Atomic Structure and Chemical Bonding
Solutions for Chemistry, Class 9, ICSE
Exercise 4A
20 questionsAnswer:
(a) The idea of the smallest unit of matter was given by Maharshi Kanada. According to him, matter consisted of indestructible minute particles called paramanus, now called atoms. A paramanus does not exist in free state, rather it combines with other paramanus to form a bigger particle called anu, now known as molecule. There are different types of paramanus. Each one of them exhibits specific properties.
(b) Democritus called the paramanu an atom, which is derived from the Greek word atomos, meaning indivisible.
Answer:
John Dalton gave the first scientific theory about the structure of matter. He considered atoms as indivisible particles that are the fundamental building blocks of matter. The existence of different types of matter is considered to be due to different types of atoms constituting them.
Main postulates of Dalton's atomic theory are:
- Matter consists of very small and indivisible particles called atoms.
- Atoms can neither be created nor be destroyed.
- The atoms of an element are alike in all respects, but they differ from the atoms of other elements.
- Atoms of an element combine in small numbers to form molecules.
- Atoms of one element combine with atoms of another element in a simple ratio to form molecules of compounds.
- Atoms are the smallest units of matter that can take part in a chemical reaction.
Answer:
Cathode rays are a stream of electrons that travel from the negatively charged electrode (the cathode) to the positively charged electrode (the anode) in a discharge tube.
Formation of cathode rays — When a high voltage (10,000 volts) charge from an induction coil is applied to tubes filled with gases at very low pressure (0.01 mm of Hg), the gases become good conductors of electricity and begin to flow from cathode to anode in the form of rays which are called cathode rays.
Answer:
In discharge tube experiment, Goldstein noticed another set of rays travelling in a direction opposite to that of the cathode rays i.e., from anode towards cathode, when perforated cathode was used in the discharge tube. He called these rays as canal rays since these rays passed through the holes or canals in the cathode. These rays were named positive rays or anode rays.
Answer:
Cathode rays | Anode rays |
---|---|
They travel from the cathode to the anode. | They travel from the anode to the cathode. |
They cause a greenish-yellow fluorescence on a soda-glass screen. | They produce fluorescence on a zinc sulphide screen. |
They are made of negatively charged particles. | They are made of positively charged particles. |
They are affected by electric field i.e., they are inflected towards a positive field and deflected from the negative field. This shows they carry negative charge. | They are deflected by electric and magnetic fields but in a direction opposite to that of cathode rays. This means that these rays consist of positively charged particles called protons. |
The ratio of charge (e) to the mass (m) remains same irrespective of the nature of the gas taken and of the metal forming the cathode. | The ratio of charge (e) to the mass (m), differs from gas to gas. |
Answer:
Discovery of sub-atomic particles i.e., electrons, protons and neutrons shows that atoms are divisible.
In discharge tube experiment, Goldstein noticed, set of rays travelling in a direction opposite to that of the cathode rays. These were called canal rays since these rays passed through the holes or canals in the cathode. These rays were named positive rays or anode rays. These rays consist of sub-atomic particle proton.
Answer:
(a) Hydrogen does not contain neutron.
(b) If an atom contains one electron and one proton, it will not carry any charge i.e., it will be a neutral atom.
Answer:
According to Thomson's atomic model —
- An atom is considered to be a sphere of uniform positive charge and electrons are embedded into it.
- The total positive charge is equal to the total negative charge so that an atom as a whole is electrically neutral.

Answer:
(a) Rutherford selected gold for his α-particle scattering experiment as he wanted as thin a layer as possible and gold is the most malleable metal.
(b) (i) Heavy nucleus like platinum will show the same observation with α-particles as shown by gold foil.
- Most of the α-particle will pass straight.
- Some α-particle will slightly deflect from their straight path.
- Very few will be deflected by very large angles or completely bounce back.
(ii) If light nuclei like lithium is used then fast moving α-particles may even push the light nucleus aside and may not be deflected back.
Answer:
According to Rutherford's model of an atom, there is a positively charged mass in the centre of the atom, i.e., nucleus, in which the entire mass of the atom is concentrated, which makes nucleus the densest part of the atom. The electrically positive nature of the nucleus is supported by the fact that the positively charged α-particles approaching the nucleus get deflected. This proved that nucleus contained protons.
Answer:
Rutherford through an experiment, directed a stream of α-particles towards a very thin gold foil. Most of the α-particle pass through the metal foil without deviating from their path. This showed that an atom contains a large empty space (extra nuclear space).
Answer:
(a) A is First energy level (K shell)
B is Second energy level (L shell)
C is Third energy level (M shell)
Shell K has the minimum amount of energy.
(b) X is the nucleus, it is positively charged.
(c) The above sketch is of Bohr's atomic model model of an atom.
Answer:
Postulates of Bohr's atomic model are as follows
- The electrons revolving around the nucleus are confined to certain fixed orbits called shells or energy levels, each of which is associated with a fixed amount of energy.
- While it is revolving around the nucleus in an orbit, an electron neither loses nor gains energy.
- An electron revolving in a particular orbit, on gaining a certain amount of energy, jumps to the next orbit and vice verse. Since each orbit is associated with a fixed amount of energy, Bohr called it an energy level.
Exercise 4B
16 questionsAnswer:
(a) The three fundamental particles of an atom are:
- electrons
- protons
- neutrons
(b) The table below shows the symbol and charge of the particles:
Particle | Symbol | Charge |
---|---|---|
Electron | 0-1e or e- | -1 |
Proton | 11H or p+ | +1 |
Neutron | 10n or n | No charge |
Answer:
Element | Symbol | No of Protons | No of Neutrons | No of Electrons |
---|---|---|---|---|
Sodium | 2311Na | 11 | 12 | 11 |
Chlorine | 3517Cl | 17 | 18 | 17 |
Uranium | 23892U | 92 | 146 | 92 |
Fluorine | 199F | 9 | 10 | 9 |
P — 12
Q — 17
R — 23892U
S — Fluorine
Answer:
Atomic number of sodium is 11 implies that number of protons in the nucleus of sodium = number of electrons in a neutral atom of sodium = 11
Mass number of sodium is 23 implies that the sum of protons and neutrons in the nucleus of sodium = 23
Answer:
p = proton
n = neutron
e = electron
Particle | Symbol | Superscript Charge (electronic unit) | Subscript mass (amu) |
---|---|---|---|
Electron | -1e0 | -1 | = 0 |
Proton | 1p1 | +1 | 1 |
Neutron | 0n1 | 0 | 1 |
Answer:
Distribution of electrons into various orbits was suggested by Bohr and Bury
- The maximum possible number of electrons in a particular shell is given by the formula 2n2, where n is the serial number of that shell.
First shell or K shell has (2 x 12) = 2 electrons
Second shell or L shell has (2 x 22) = 8 electrons
Third shell or M shell has (2 x 32) = 18 electrons
Fourth shell or N shell has (2 x 42) = 32 electrons
and so on. - Electrons are not accommodated in a given shell unless the inner shells are filled. In other words the shells are filled in stepwise manner.
- The outermost shell of an atom cannot accommodate more than 8 electrons, even if it has the capacity to accommodate more than 8 electrons.
- The outermost shell of a chemically stable atom can accommodate a maximum of 8 electrons, except Hydrogen and Helium atoms, which have only one shell, and can thus accommodate a maximum of 2 electrons only.
Answer:
Atomic number is equal to number of electrons.
M shell is the third energy level, K and L shell are completely filled.
The electronic configuration
K shell - 2 electrons
L shell - 8 electrons
M shell - 3 electrons
Total no. of electron = 2 + 8 + 3 = 13
Hence, total number of electron is 13. So, the atomic number of the element is 13.
Answer:
The most stable atom from the given list is (Argon).
Reason — Argon is a noble gas with a completely filled outermost shell, making it chemically inert and highly stable.
In contrast, the other atoms, , , , have incomplete outer shells and are therefore reactive as they tend to gain, lose, or share electrons to achieve stability.
Answer:
(a) Atomic number = Number of electrons
Hence, atomic number is 9
(b) Mass number = Number of protons + Number of Neutrons
Mass number = 9 + 10 = 19
Hence, mass number is 19
(c) Electronic configuration - 2, 7
K shell - 2 electrons
L shell - 7 electrons
Answer:
Given, Atomic number = 16 and mass number = 32
So, Number of protons = 16
Number of electrons = 16
Number of neutrons = 32 – 16 = 16
Electronic configuration = 2, 8, 6
Below diagram shows the arrangement of the electrons in an atom of sulphur:

Answer:
(a) electronic configuration of 1327X is [2, 8, 3]
(b) electronic configuration of 1735Y is [2, 8, 7]
Number of electrons in X = 13
Mass number of Y = 35 and number of protons in Y = 17
∴ Number of neutrons in Y = 35 - 17 = 18
Exercise 4C
39 questionsAnswer:
Atomic nucleus
Reason — Rutherford through an experiment, directed a stream of α-particles towards a very thin gold foil. Most of the α-particles passed through the metal foil without deviating from their path. Very few (nearly one in ten thousand) alpha particles were either deflected by very large angles or completely bounced back. This suggested the presence of a positively charged mass at the centre of the atom, known as the nucleus.
Answer:
3115A
Reason — Atomic number is 15 so it will be written in subscript and mass number = 15 + 16 = 31 so it will be written in superscript. Hence the correct representation will be 3115A
Answer:
Can be metals or non-metals
Reason — If the element shows positive valency it is a metal (e.g., Beryllium) and if the element shows negative valency it will be a non-metal (e.g., Sulphur).
Answer:
Protons and neutrons
Reason — The nucleus is located at the centre of the atom, it contains protons and neutrons, which account for the total mass of that atom.
Answer:
Noble gas
Reason — Elements tend to combine with one another to attain the stable electronic configuration of the nearest noble gas i.e., 2 or 8 electrons in its valence shell.
Name the following:
(a) The element which does not contain any neutron in its nucleus.
(b) An element having valency 'zero'.
(c) Metal with valency 2
(d) Two atoms having the same number of protons and electrons but different number of neutrons.
(e) The shell closest to the nucleus of an atom.
Answer:
(a) Hydrogen
(b) Helium
(c) Magnesium
(d) Isotopes
(e) K shell
Answer:
Property | Atomic Number |
---|---|
A solid non-metal of valency 3 | 15 |
A gas of valency 2 | 8 |
A metal of valency 1 | 19 |
A non-metal of valency 4 | 14 |
(a) As atomic number 15 has electronic configuration [2,8,5], so it is a non-metal having 5 electrons in the valence shell and needs three electrons to complete its octet so it has a valency 3.
(b) As atomic number is 8, so it has the electronic configuration of [2,6] and it needs 2 electrons to complete its octet hence, it is a gas of valency 2
(c) As atomic number 19 has electronic configuration [2,8,8,1], so it is a metal having 1 electron in the valence shell and donates its one electron to complete its octet so it has a valency 1.
(d) As atomic number 14 has electronic configuration [2,8,4], so it is a non-metal having 4 electrons in the valence shell and needs four electrons to complete its octet so it has a valency 4.
Answer:
(a) Elements with the valence shell having 8 electrons and helium with 2 electrons, cannot gain or lose electrons so they are chemically inactive and are called inert elements.
(b) Inert elements exist as monoatoms because they have a completely filled outermost electron shell. This makes them very stable and unlikely to form chemical bonds with other atoms. Hence, they tend to exist as monoatoms in molecules rather than participating in chemical reactions.
(c) Electrons present in the outermost shell (i.e., valence shell) of an atom are valence electrons.
Answer:
Hydrogen has three isotopes i.e., namely protium (11H), deuterium (21H) and tritium (31H) each having the same atomic number i.e., 1 but different mass numbers i.e., 1, 2 and 3 respectively.
Hence, all three have same number of electrons and protons whereas number of neutrons are 0, 1 and 2 respectively.
Isotopes of Hydrogen

Answer:
As elements are arranged on the basis of the number of protons in their respective atoms, therefore, proton helps in determining and understanding the structure and behavior of an atom. Protons also determine the positive charge of the nucleus in the atom
Answer:
(a) X+
Reason — Since the element X has 1 electron in its valence shell. So, it will lose an electron and gain noble gas configuration. Therefore it forms a cation (X+). The sign is positive and the charge is of 1 proton.
(b) X will be a strong reducing agent as it has the tendency to donate its valence electron.
Answer:
(a) Mass number (A) — The mass number of an atom is equal to the total number of nucleons (i.e., the sum of the number of protons and the number of neutrons ) in it's nucleus. It is denoted by A.
A = number of protons + number of neutrons
Mass number however is a whole number approximation of the atomic mass calculated in atomic mass units.
(b) Ion — An ion is a charged particle which is formed due to the gain or loss of one or more electrons by an atom.
For example, a sodium ion (Na+) is formed when a sodium atom loses one electron and Chlorine ion (Cl-) ion is formed when a chlorine atom gains an electron.
(c) Cation — A metallic atom, which loses electron(s), becomes a positively charged ion and is known as a cation.
(d) Anion — A non-metallic atom, which gains electron(s) becomes a negatively charged ion and is known as an anion.
(e) Element — An element is a pure substance that can neither be formed from, nor decomposed into simple substances by ordinary physical or chemical methods.
(f) Orbit — The imaginary circular path around the nucleus of an atom, in which the electrons revolve.
Answer:
(a) Number of electrons in the M shell = 2. So, the electrons in the K and L shells = 2 and 8
Hence, atomic number = 2 + 8 + 2 = 12
(b) As number of protons = number of electrons = 12
Answer:
(a) (i) Atom E.
As its atomic number is given as 7, therefore it contains 7 protons.
(ii) Atom B.
As its atomic number is 9, hence it has 9 electrons, therefore, its electronic configuration is 2, 7.
(b) Atom C has electronic configuration : [2, 1]
Atom D has electronic configuration : [2, 6]
As D requires two electrons to complete its octet, hence, it combines with two atoms of C which donate their valence electrons to attain stable noble gas configuration and D accepts the two electrons and attains a stable state, hence the formula of the compound formed between C and D is C2D
(c) (i) Metals: A [2,8,8,2] and C [2,1]
Reason — As the elements A and C have 2 and 1 electrons respectively in the valence shells hence, they are metals.
(ii) Non-metals: B [2,7], D [2,6], E [2,5].
Reason — As the elements B,D and E have 7,6 and 5 electrons respectively in the valence shell, hence they are non-metals.
Answer:
(a) (i) The composition of their nuclei :
:
Atomic number [Z] = electron [e] = proton [p] = 12
Mass number [A] = 24
As, neutron [n] = A - Z = 24 - 12 = 12.
Hence, p = 12, n = 12
:
Atomic number [Z] = electron [e] = proton [p] = 12
Mass number [A] = 26
As, neutron [n] = A - Z = 26 - 12 = 14.
Hence, p = 12, n = 14
(ii) Their electronic configurations :
:
As electron [e] = 12 hence, electronic configuration = 2,8,2
:
As electron [e] = 12 hence, electronic configuration = 2,8,2
(b) Reason for difference in mass numbers — As the number of neutrons is different in the two isotopes of magnesium, therefore, they have different mass numbers.
Answer:
Constituents of the nucleus are called nucleons, i.e. protons and neutrons.
3115P has atomic number 15, hence it has 15 protons.
Mass number 31, hence number of neutrons = 31 - 15 = 16
Therefore, number of nucleons are p + n = 15 + 16 = 31.
Structure of Phosphorus is shown in the diagram below:

Answer:
Isotopes are atoms of the same element having same atomic number but different mass number.
For example : 2412Mg and 2612Mg
Isotopes differ in the number of neutrons.
Uses of isotopes
- Radioactive isotopes are used in industry to detect the leakage in underground oil pipelines, gas pipelines and water pipelines.
- An isotope of Uranium 23592U is used as a fuel in nuclear reactors.
Answer:
Isotopes have the same atomic number i.e., the same number of electrons and protons and hence the same electronic configuration. Chemical properties are determined by the electronic configuration of an atom. Thus, the isotopes of an element are chemically alike.
Isotopes have different mass numbers [A] due to different number of neutrons. 3517Cl has 18 neutrons whereas 3717Cl has 20 neutrons. Due to different atomic masses these atoms differ in their physical properties like density, melting point, boiling point, etc.
Answer:
Fractional atomic mass is the weighted average of the atomic masses of all the naturally occurring isotopes of that element.
Fractional mass of chlorine is 35.5.
Any natural sample of chlorine contains 75% of 3517Cl and 25% 3717Cl, i.e., these isotopes are in the ratio of 3:1 respectively. Thus, average atomic mass of chlorine is calculated as
= 35.5. Therefore, atomic mass of chlorine atom is 35.5
Answer:
(a) Atomic number of an element is the number of:
- electrons present in its neutral atom.
- protons present in the nucleus of its atom.
- positive charge in the nucleus of its atom.
- it is denoted by Z.
(b) Complete the table given below:
No of protons | No of electrons | No of neutrons | Atomic number | Mass number | |
---|---|---|---|---|---|
3517Cl | 17 | 17 | 18 | 17 | 35 |
3717Cl | 17 | 17 | 20 | 17 | 37 |
(c) Write down the electronic configuration of
(i) Chlorine atom — 2, 8, 7
(ii) Chloride ion — 2, 8, 8
Answer:
(a) Physical properties depend on the atomic mass (mass number) and isotopes have different mass numbers due to different number of neutrons hence they differ in their physical properties.
(b) Argon [2, 8, 8] does not react or do not combine either with other atoms or among themselves to form molecules, since their octet is already complete, hence, they are in a stable state and their combining capacity is zero.
(c) Actual atomic mass is greater than the mass number because the mass number is a whole number approximation of atomic mass unit. Actual atomic mass is fractional as it is the weighted average of all the naturally occurring isotopes of that element.
(d) Chemical properties are determined by the electronic configuration of an atom and electronic configuration of both isotopes of chlorine is same, i.e., electronic configuration of 3517Cl is [2,8,7] and electronic configuration of 3717Cl is [2,8,7], hence they both have same chemical properties.
Answer:
Element Symbol | Atomic Number | Mass Number | Number of neutrons | Number of electrons | Number of protons |
---|---|---|---|---|---|
Li | 3 | 6 | 3 | 3 | 3 |
Cl | 17 | 37 | 20 | 17 | 17 |
Na | 11 | 23 | 12 | 11 | 11 |
Al | 13 | 27 | 14 | 13 | 13 |
S | 16 | 32 | 16 | 16 | 16 |
Answer:
(a) Radioactive isotopes — Used in industry to detect the leakage in underground oil pipelines, gas pipelines and water pipelines.
(b) 235U — Used as fuel in nuclear reactors.
(c) 60Co — used in radiotherapy for treating cancer and other diseases.
(d) 131I — Used in treatment of goitre.
(e) 14C — Used for determining the age of historical and geological material.
Answer:
Contradictions with Modern atomic theory:
Dalton's atomic theory | Modern atomic theory |
---|---|
Atoms are indivisible particles. | Atoms are divisible into sub-atomic particles like protons, electrons and neutrons. |
Atoms can neither be created nor be destroyed. | Atoms can be created and destroyed by nuclear fusion and fission. |
The atoms of an element are alike in all respects, but they differ from the atoms of other atoms. | The atoms of an element may not be alike in all respects, as seen in the case of isotopes. |
Atoms of elements combine in small whole numbers to form molecules. | In the formation of organic compounds, the numbers can be very big such as thousands. |
Correlation with Modern atomic theory:
The modern atomic theory however correlates with Dalton's atomic theory in the fact that atoms are the smallest unit of matter taking part in a chemical reaction and in a given compound the relative number and kind of atoms is a constant.
Answer:
(a) For an atom to achieve stable electronic configuration it must have 8 electrons in outermost orbit like that of noble gases other than Helium. This is known as Octet rule.
(b) For an atom of hydrogen to achieve stable electronic configuration it must have 2 electrons in the first shell [outermost] like that of noble gas Helium. So, two hydrogen atoms share one electron each to become stable. This is known as Duplet rule.
An element
A has atomic number 7, mass number 14
B has electronic configuration 2, 8, 8
C has 13 electrons, 14 neutrons
D has 18 protons 22 neutrons
E has electronic configuration 2, 8, 8, 1
State
(i) valency of each element
(ii) which one is a metal
(iii) which is a non-metal
(iv) which is an inert gas.
Answer:
(i) Valency of each element:
Element A : Atomic number (number of electrons) = 7
Electronic configuration = 2, 5
Valency of A = 8 - 5 = 3
Element B : Electronic configuration 2, 8, 8
Valency of B = Zero
Element C : has 13 electrons
Electronic configuration = 2, 8, 3
Valency of C = 3
Element D : Protons = Electrons = 18
Electronic configuration = 2, 8, 8
Valency of D = Zero
Element E : Electronic configuration = 2, 8, 8, 1
Valency of E = 1
(ii) C and E are metals.
(iii) A is a non-metal.
(iv) B and D are inert gases.
Exercise 4D
13 questionsAnswer:
Bonds formed between metals and non-metals are electrovalent bonds.
Metals have 1, 2 or 3 electrons in their valence shell. They have a tendency to lose their valence electrons. So they combine with non-metals which have 5, 6 or 7 electrons in their valence shell and thus have a tendency to gain electron(s) and form electrovalent bonds.
Answer:
(a) Since there are three electrons more than the noble gas configuration in the atom so the atom will try to lose three electron to gain noble gas configuration. Therefore, it will form a cation X3+.
Formula of its:
(i) Sulphate — X2(SO4)3
(ii) Nitrate — X(NO3)3
(iii) Phosphate — XPO4
(iv) Carbonate — X2(CO3)3
(v) Hydroxide — X(OH)3
Answer:
The basic tendency of an atom which makes it to combine with other atoms is the tendency of elements to acquire the nearest noble gas configuration in their outermost orbit and become stable
Answer:
Compounds formed between metals and non-metals are electrovalent compounds.
The metals have 1, 2 or 3 electrons in their valence shell. They have a tendency to lose their valence electrons. So they combine with non-metals which have five, six or seven electrons in their valence shell and thus have a tendency to gain electron(s) and form electrovalent compounds.
Answer:
(a) Nature of bond is ionic because the atom X gives one electron to each Y atom.
(b) Orbit structure of XY2 :

Answer:
(a) Ionic Bond
Reason — Since X has 7 electrons in its outermost shell and Y has only one electron in its outermost shell. So, Y loses its one electron and X gains that electron to form an ionic bond.
(b) The formula of the compound is XY.
Answer:
(a) Comparison between sodium atom and sodium ion
Property | Sodium Atom | Sodium Ion |
---|---|---|
Atomic Structure | It has one electron in M shell. | It has 8 electrons in L shell. |
Electrical State | It is neutral. | It is positively charged. |
(b) Comparison between chlorine atom and chloride ion
Property | Chlorine Atom | Chloride Ion |
---|---|---|
Atomic Structure | It has 7 electrons in M shell. | It has 8 electrons in M shell. |
Electrical State | It is neutral. | It is negatively charged. |
Answer:
The electronic configuration of Fluoride ion is the same as that of a neon atom i.e., both have 8 electrons in their valence shell but Fluoride attain that configuration by gaining an electron so it is negatively charged but neon atom have its own 8 valence electrons so it is neutral i.e., it has no charge on it.
Answer:
(a) Redox Reaction — A reaction in which oxidation and reduction occur simultaneously is called an oxidation-reduction reaction or redox reaction.
In redox reaction there is a transfer of electrons between two atoms.
(b) The reaction in which electron(s) is gained is called a reduction reaction and the reaction in which there is a loss of electron(s) is called an oxidation reaction
Answer:
The electronic configuration of:
Potassium (19) = 2, 8, 8, 1
Chlorine (17) = 2, 8, 7
The reaction is :
2K + Cl2 ⟶ K+ + 2Cl-
(i) Oxidation : 2K ⟶ K+
Potassium undergoes oxidation reaction as it loses an electron.
(ii) Reduction : Cl2 ⟶ 2Cl-
Chlorine undergoes reduction reaction as it gains an electron.
(iii) Chlorine acts as an oxidising agent and gets reduced.
(iv) Potassium acts as a reducing agent and gets oxidised.
Exercise 4E Descriptive Type
6 questionsAnswer:
A single covalent bond is formed by sharing of one pair of electrons between the atoms, each atom contributing one electron.
Ex: Molecule of hydrogen [H-H]
A double covalent bond is formed by sharing of two pairs of electrons between two atoms.
Ex: Molecule of oxygen [O=O]
A triple covalent bond is formed by sharing of three pairs of electrons between two atoms.
Ex: Molecule of nitrogen [N≡N]
Answer:
Magnesium loses its two valence electrons to acquire a stable octet. The two chlorine atoms accept one each of these electrons to acquire a stable octet. Thus, electrovalent bond in MgCl2 is formed by transfer of electrons.
When a molecule of CCl4 is to be formed, one atom of carbon shares four electron pairs, one with each of the four atoms of chlorine. Thus, covalent bond is formed by sharing of electrons.
Answer:
The bond formed between metal (M) and non-metal (chlorine) is ionic bond.
As M bonds with two chlorine atoms having electronic configuration [2,8,7], so we can say that M donates two electrons to two chlorine atoms to complete their octet and therefore, it has a valency of 2.
When metal (M) having +2 valency combines with sulphur [2,8,6] having -2 valency, M donates its two valence electrons to sulphur and formula of compound formed is MgS
When metal(M) having +2 valency combines with oxygen [2,6] having -2 valency, M donates its two valence electrons to oxygen and formula of compound formed is MgO
When metal (M) atoms having +2 valency combines with nitrogen [2,5] having -3 valency then an ionic bond is formed by the transfer of 6 electrons from 3 metal (M) atoms to 2 nitrogen atoms, hence the formula of the compound is Mg3N2.
Explain the following :
(a) Mass of an atom is concentrated inside the nucleus of the atom.
(b) Atoms combine by transfer and sharing of electron(s).
(c) An element has atoms with different mass number.
(d) Carbon-12 and carbon-14 both show similar chemical properties
Answer:
(a) Atom is made of primarily three sub-atomic particles namely electrons, protons and neutrons. An electron has negligible mass, and protons and neutron lie inside the nucleus, hence, mass of an atom is majorly present in the nucleus.
(b) Elements tend to combine with one another to attain the stable electronic configuration of the nearest noble gas. This is achieved either by gaining electrons or losing electrons or sharing of electrons with other atoms of the same element or another element to form a chemical compound.
(c) Isotopes are the elements having the same atomic number but different mass numbers.
For example : 2412Mg and 2612Mg
Isotopes differ in the number of neutrons.
(d) Chemical properties are determined by the electronic configuration of an atom and electronic configuration of both isotopes of carbon is same, i.e., electronic configuration of C-12 is [2,4] and electronic configuration of C-14 is [2,4], hence they both have same chemical properties.
Answer:
(a) Ethene molecule [C2H4] — It has one double covalent bond between the 2 Carbon atoms and four single covalent bonds with hydrogen atoms.

(b) Ethyne molecule [C2H2] — It has one triple covalent bond between the 2 Carbon atoms and 2 single covalent bonds with hydrogen atoms.

Exercise 4E Multiple Choice Type
25 questionsAnswer:
Electrovalent
Reason — Metals become positively charged when they give away electrons and non-metals become negatively charged when they accept electrons. Due to electrostatic attraction between the two oppositely charged ions electrovalent (ionic) bonds are formed.
Answer:
2, 8, 8
Reason — The number of electrons in calcium atom is 20 and the electronic configuration of a neutral calcium atom is 2, 8, 8, 2. Ca2+ has two electrons less so the electronic configuration becomes 2, 8, 8.
Answer:
Redox reaction
Reason — In the formation of an electrovalent bond, the transfer of electron(s) is involved. The electropositive atom loses electrons and undergoes oxidation, while the electronegative atom gains electrons and undergoes reduction. This is known as redox reaction.
Answer:
Solids
Reason — Ionic compounds have strong electrostatic forces of attraction between their ions, which cannot be separated easily. Thus, they are hard, crystalline solids consisting of ions.
Answer:
Oxygen molecule
Reason — To attain the stable electronic configuration of the nearest noble gas neon, oxygen needs two electrons.
When two oxygen atoms come closer, each contributes two electrons and so they have two shared pair of electrons between them. Both atom attain an octet, resulting in the formation of a double covalent bond [O=O] between them.
Answer:
Hydrogen chloride
Reason — The covalent bond is said to be polar when the shared pair of electrons are not at equal distance between the two atoms. This results in the development of fractional positive charge and negative charge on them and they ionise in water. For example: hydrogen chloride.
Answer:
Nitrogen
Reason — To attain the stable electronic configuration of the nearest noble gas neon, nitrogen needs three electrons.
When two nitrogen atoms come closer, each contributes three electrons and so they have three shared pair of electrons between them. Both atom attain an octet, resulting in the formation of a triple covalent bond [N≡N] between them.
Answer:
Neon and argon
Reason —
Atoms involved | Electronic configuration | Nearest noble gas |
---|---|---|
Carbon | C [2,4] | Neon [2,8] |
Chlorine | Cl [2,8,7] | Argon [2,8,8] |
Answer:
It is formed between metals and non-metals
Reason — Bonds formed between metals and non-metals are electrovalent bonds.
Metals have 1, 2 or 3 electrons in their valence shell. They have a tendency to lose their valence electrons. So they combine with non-metals which have 5, 6 or 7 electrons in their valence shell and thus have a tendency to gain electron(s) and form electrovalent bonds.
Answer:
It loses electrons and is oxidized.
Reason — As oxidation is loss of electrons so the metal atom is oxidised after losing electrons.
Answer:
XY2
Reason — As element X has 2 electrons in its 2nd orbit, so the electronic configuration of X is [2, 2] and Y has 7 electrons in the third orbit so its electronic configuration as [2, 8, 7]
X donates it two valence electrons and completes its octet and two atoms of Y accept the electrons to attain the octet, hence the compound formed is XY2.
Answer:
Both Q and R
Reason — An atom which is neutral contains neutrally charged neutrons and positively charged protons [collectively called nucleons] in the central part or nucleus of the atom.
Answer:
Only P
Reason — According to mordern atomic theory, atoms of an element may not be alike in all respects, as is seen in the case of isotopes.
Modern atomic theory states that atoms can be created and destroyed by nuclear fusion and fission and in the formation of organic compounds, the number of atoms that combine is very big such as thousands. Hence, other two statements are incorrect.
Answer:
Only R
Reason — An atom as a whole is electrically neutral, because the total positive charge of the nucleus is balanced by the total negative charge of the electrons, i.e., the number of protons and electrons in an atom are equal.
Answer:
Both Q and R
Reason — Magnesium has two valence electrons in its outermost orbit. So, it will donate its electrons and undergo oxidation to obtain octet configuration. Each chlorine has 7 valence electrons in its outermost orbit. So, it will gain one electron each and undergo reduction to obtain octet configuration.
Select the correct statement(s) for the diagram shown below.

P — One atom of carbon transfers one electron to each chlorine atom.
Q — One atom of carbon shares four electron pairs, one with each of the four atoms of chlorine.
R — Carbon atom attains neon configuration and chlorine attains argon configuration after the combination.
- Only P
- Only Q
- Only R
- Both Q and R
Answer:
Both Q and R
Reason — Carbon has the electronic configuration of [2, 4] and Chlorine has 7 electrons in the third orbit so its electronic configuration is [2, 8, 7]. Carbon forms covalent bond by sharing four electron pairs, one with each of the four atoms of chlorine. After sharing of electrons in order to obtain stable octate configuration, carbon attains electronic configuration of neon as [2, 8] and chlorine attains electronic configuration of argon as [2, 8, 8].
Assertion (A): The atomic mass of an element is mainly the mass of its nucleus.
Reason (R): The nucleus of an element contains neutrons and protons and the electrons have a negligible mass.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
Answer:
Both A and R are true and R is the correct explanation of A.
Explanation — Mass number of an element is the total number of protons and neutrons present in its nucleus. Hence, the asertion (A) is true.
Protons and neutrons each have a mass of about 1 atomic mass unit (amu). Electrons have a mass approximately 1/1836 that of a proton, extremely small and usually neglected in atomic mass calculations. Hence reason (R) is true and reason (R) is the correct explanation of assertion (A).
Assertion (A): An alpha particle is the same as a Helium nucleus.
Reason (R): An alpha particle does not have any electron.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
Answer:
Both A and R are true but R is not the correct explanation of A.
Explanation — An alpha particle contains two protons and two neutrons, same as helium nucleus. Hence the assertion (A) is true
An alpha particle is formed by removing 2 electrons from helium atom. So, it does not have any electrons. Hence the reason (R) is true.
But reason does not explain why alpha particle is same as helium nucleus. Hence reason (R) is not correct explanation of assertion (A).
Assertion (A): Isotopes of an element have similar chemical properties.
Reason (R): Isotopes of an element have the same atomic number.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
Answer:
Both A and R are true and R is the correct explanation of A.
Explanation — Isotopes have the same atomic number (Z), i.e., the same number of electrons and protons and hence the same electronic configuration. Chemical properties are determined by the electronic configuration of an atom. Thus, isotopes of an element are chemically alike. Hence both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
Assertion (A): Al3+ ion is isoelectronic with Neon.
Reason (R): Both have the same number of electrons.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
Answer:
Both A and R are true and R is the correct explanation of A.
Explanation — Aluminium (Al) has electronic configuration 2, 8, 3. Al3+ is formed when Aluminium loses 3 electrons making its electronic configuration 2, 8. The electronic configuration of neon is also 2, 8. So, Al3+ and Ne have the same number of electrons i.e., they are isoelectronic. Hence both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
Assertion (A): Neon is a noble gas.
Reason (R): Neon is not very reactive due to its small size.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
Answer:
A is true but R is false.
Explanation — Neon is a noble gas with stable electronic configuration of 2, 8. Hence the assertion (A) is true.
Neon is stable and not reactive because of its electronic configuration which satisfies octet rule, not because of its small size. Hence the reason (R) is false.
Assertion (A): The properties of an ion are different from that of the parent atom.
Reason (R): Atoms and ions differ with respect to the number of neutrons and protons in them.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
Answer:
A is true but R is false.
Explanation — When an atom changes into an ion, its electrical charge changes, affecting its conductivity, ionic radius, and other physical and chemical characteristics. Hence the assertion (A) is true.
When an atom changes to ion, number of protons and number of neutrons remain same. But number of electrons change. It will either increase or decrease. Hence reason (R) is false.
Assertion (A): The valency of an element is its combining capacity with an atom of another element.
Reason (R): Elements take part in chemical combination to acquire the electronic configuration of the nearest noble gas.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
Answer:
Both A and R are true but R is not the correct explanation of A.
Explanation — Valency of an element is the combining capacity of that element with other elements and is equal to the number of electrons that take part in a chemical reaction. Hence the assertion (A) is true. The atoms of all elements, other than the inert gases, combine to attain a stable configuration of nearest noble gas. Hence the reason (R) is true.
However, reason (R) is not the correct explanation of assertion (A), because valency is defined by how many electrons are involved in bonding, not by why the atom bonds (which R explains). Hence, while both statements are correct individually, reason (R) doesn’t directly explain assertion (A).
Exercise 4E Numericals
2 questionsAnswer:
Let,
the percentage of 168A be X and
percentage of 188A be 100-X
Average atomic mass = = 16.2
⇒ 16X + 1800 - 18X = 1620
⇒ -2X + 1800 = 1620
⇒ -2X = 1620 - 1800
⇒ 2X = 180
⇒ X = = 90
∴ 100 - X = 100 - 90 = 10
Hence, percentage of the two isotopes in this sample are 90% of 168A and 10% of 188A
Exercise 4E Short Answer Type
9 questionsAnswer:
(a) Covalent Bond — The chemical bond that is formed between two combining atoms by mutual sharing of one or more pair of electrons is called a covalent bond. Depending on the number of electron pairs shared : bond is single [-], double [=], or triple [≡] covalent.
(b) (i) Bond formed between two H atoms; H-H
A hydrogen atom has one electron in its only shell. It needs one more electron to attain duplet. To meet this need, hydrogen atom shares its electron with another hydrogen atom. Thus, one electron each is contributed by the two atoms of hydrogen, and the resulting pair of electrons is mutually shared by both the atoms to form a hydrogen molecule.
(ii) Bond formed between a hydrogen atom and chlorine atom; H‒Cl
In the case of hydrogen chloride molecule, hydrogen atom shares an electron pair with chlorine atom, such that hydrogen acquires a duplet configuration and chlorine an octet, resulting in the formation of a single covalent bond.
Answer:
Element A with 1 electron in its first shell is hydrogen, and element B with 7 electrons in its third shell is chlorine. So, a single covalent bond is formed between hydrogen and chlorine by sharing one pair of electron.
Answer:
(a) X[6] = Electronic configuration [2,4]
Y[9] = Electronic configuration [2,7]
Z[12] = Electronic configuration [2,8,2]
(i) Y gains one electron to attain a stable octet, hence becomes negatively charged anion.
(ii) Z loses two electrons to attain a stable octet hence becomes positively charged cation.
(b) Ionic bond with molecular formula ZY2.
Answer:
Potassium chloride being an electrovalent compound consists of ions. In aqueous solution state, the electrostatic forces between them weaken and the ions become mobile conducting electricity.
On the other hand, hydrogen chloride is a polar covalent compound. The shared electron pair is shifted towards Chlorine thereby developing a slight negative charge on it and a slight positive charge on hydrogen atom. In its aqueous solution state, hydrogen chloride ionises into Hydronium and Chloride ions as shown in the equation below. These ions help in conducting electricity.
HCl + H2O ⇌ H3O+ + Cl-
Answer:
With Chlorine — MCl2
With Sulphur — MS
Explanation:
Given, M reacts with oxygen to form an ionic bond MO. Oxygen needs 2 electrons to complete its octet forming the anion O2-. As the compound formed is MO, it means that M has 2 electrons in its valence shell which it donates to Oxygen to attain a stable electronic configuration forming a cation M2+.
Each Chlorine atom accepts 1 electron to complete its octet so 2 Chlorine atoms combine with 1 atom of M to form MCl2. Each Sulphur atom accepts 2 electrons to complete its Octet so 1 Sulphur atom combines with 1 atom of M to form MS.
Answer:
Covalent bonding
Reason — The valency of hydrogen element is 1 and that of oxygen is 2. Hydrogen needs one electrons to attain stable duplet structure of nearest noble gas - He [2] and oxygen needs two electrons to attain stable octet structure of nearest noble gas - Ne [2,8]
Each of the two hydrogen atoms shares an electron pair with the oxygen atom such that hydrogen acquires a duplet configuration and oxygen an octet configuration resulting in the formation of two single covalent bonds [H-O-H] in the molecule of water.

(ii) Electrovalent bonding
Reason — The valency of calcium (2,8,8,2) element and that of oxygen (2,6) is 2. Calcium donates two electrons and oxygen takes up those two electrons and electrovalent bond is formed.
(iii) Polar covalent bonding Reason — In hydrogen chloride, the strong nuclear charge of the chlorine atom attracts the shared electron pair towards itself, i.e., negative charge shifts towards the chlorine atom thereby developing a slight negative charge on it. The hydrogen atom develops a slight positive charge. Therefore, a polar covalent bond is formed as shown below :

Answer:
(a) (i) A and B are isotopes, as they have same atomic number but different mass number.
(ii) X and Y are isobars, as they have same mass number but different atomic number.
(b) (i) Examples of isotopes — 3517Cl, 3717Cl
(ii) Examples of isobars — 4018Ar, 4020Ca
Exercise 4E Structuredapplicationskill Type
6 questionsThe atom of an element A has 6 electrons in its M shell.
(a) Write the electronic configuration of element A.
(b) What is the atomic number of element A?
(c) Is it a metal or a non-metal?
(d) What type of ion will be formed by an atom of element A? Write the symbol of the ion formed.
(e) What could element A be ?
(f) Draw the orbital structure of A and its ion.
Answer:
(a) [2, 8, 6], as it has three shells — K, L, M.
(b) 16, as atomic number is the total number of electrons so, 2+8+6 = 16
(c) Non-metal, as it has 6 electrons in the valence shell and will accept 2 electrons to complete its octet.
(d) Anion, A2-, as element A has 6 valence electrons so it has a valency of -2 and it will take two electrons, complete its octet and become a negatively charged ion.
(e) Sulphur, as sulphur also has the electronic configuration of [2, 8, 6].
(f) Orbital structure of A and its ion is shown below:

An atom of an element Y may be written as 94Y.
(a) What does the figure 9 indicate?
(b) What does the figure 4 indicate?
(c) What is the number of protons in atom Y?
(d) What is the number of neutrons in atom Y?
(e) What is the number of electrons in atom Y?
(f) How many electrons are there in the outermost shell of an atom of element Y?
(g) Write the symbol of ion formed by an atom of element Y?
Answer:
(a) Mass number is indicated by figure 9, which is the sum of protons and neutrons in the nucleus of an atom.
(b) Atomic number is indicated by figure 4, which is the number of electrons present in a neutral atom.
(c) 4, as the atomic number also indicates the number of protons in the nucleus of an atom.
(d) 5, number of neutron = mass number - number of protons = 9-4=5
(e) 4, as the atomic number is 4, which is the number of electrons present in a neutral atom.
(f) 2, its electronic configuration is [2, 2]
(g) Y2+, as it has the valency of +2, hence it loses two valence electrons and becomes a positively charged ion.
Atom A is represented as 20982A, atom B as 20983B and atom C as 21182C
(a) How many electrons, protons and neutrons do A, B and C have?
(b) Which two atoms have the different number of nucleons?
(c) Name the term which can be used for A and C atoms.
(d) Define the term mentioned in part (c) above.
Answer:
(a)
Atom | Electron (e) | Proton (p) | Neutron (n = mass number - p) |
---|---|---|---|
20982A | 82 | 82 | 209 - 82 = 127 |
20983B | 83 | 83 | 209 - 83 = 126 |
21182C | 82 | 82 | 211 - 82 = 129 |
(b) A and C have different number of nucleon, because the number of protons is same in both but number of neutrons are different.
(c) Isotopes
(d) Isotopes are atoms of the same element having the same atomic number but different mass numbers.
(a) Name the charged particles which attract one another to form electrovalent compounds.
(b) In the formation of electrovalent compounds; electrons are transferred from one element to another. How are electrons involved in the formation of a covalent compound ?
(c) The electronic configuration of nitrogen is (2, 5). How many electrons in the outer shell of a nitrogen atom are not involved in the formation of a nitrogen molecule ?
(d) In the formation of magnesium chloride (by direct combination between magnesium and chlorine), name the substance that is oxidized and the substance that is reduced.
Answer:
(a) Cation [positively charged] and anion [negatively charged]
(b) In the formation of covalent compounds, the electrons are shared between the combining atoms. Depending on the number of electron pairs shared : bond is single [-], double [=], or triple [≡] covalent.
(c) Two electrons in the outer shell of a nitrogen atom are not involved in the formation of a nitrogen molecule.
Reason — Nitrogen has 5 electrons in the valence shell whereas it forms triple bond to fulfill it's octet and hence, 2 electrons are left in the outer shell of a nitrogen atom which are not involved in the formation of a nitrogen molecule.
(d) Magnesium is oxidized and Chlorine is reduced.
Reason — In the formation of magnesium chloride the reaction is :
Mg + Cl2 ⟶ Mg2+ + 2Cl-
Oxidation : Mg ⟶ Mg2+
Reduction : Cl2 ⟶ 2Cl-
An element X has 2 electrons in its M shell, it forms bond with an element Y which has 7 electrons in its third orbit.
(a) Write the formula of the compound formed.
(b) Which nearest inert gas electronic configuration will element X and Y acquire?
(c) Show using an orbital diagram the formation of compound between X and Y.
Answer:
(a) XY2
Reason — As element X has 2 electrons in its M shell, so the electronic configuration of X is [2, 8, 2] and Y has 7 electrons in the third shell so its electronic configuration is [2, 8, 7]
X donates it two valence electrons and completes its octet and two atom of Y accepts the electrons to attain the octet, hence the compound formed is XY2.
(b) The nearest inert gas electronic configuration for element X is 2, 8 i.e., Neon while that for element Y is 2, 8, 8 i.e, Argon.
(c) Orbital diagram of XY2

In the formation of (i) Oxygen molecule (ii) Carbon tetrachloride molecule, state the following:
(a) Electronic configuration of nearest inert gas attained.
(b) How many electrons are shared/transferred in bond formation?
(c) Which type of bonds these compounds form?
(d) Draw the orbital diagrams?
Answer:
(i) In the formation of Oxygen molecule:
(a) Neon (10) 2,8, because electronic configuration of oxygen is [2,6] so to attain the stable configuration of nearest noble gas Neon, oxygen needs two electrons.
(b) Two pairs of electrons are shared, and both oxygen atoms attain their octet.
(c) Double Covalent bond, as the bond between two oxygen atoms is formed by sharing of electrons.
(d) Orbital Diagram of the formation of Oxygen molecule:

(ii) In the formation of Carbon tetra chloride molecule:
Atoms involved | Electronic configuration | Nearest noble gas |
---|---|---|
Carbon | C [2,4] | Neon [2,8] |
Chlorine | Cl [2,8,7] | Argon [2,8,8] |
(b) Four pair of electrons are shared, as carbon needs four electrons and chlorine needs one electron to complete their octet, hence four chlorine atoms share their one electron each with carbon and all the atoms attain a stable configuration.
(c) Single Covalent bond, as the bond between carbon and 4 chlorine atoms is formed by sharing one pair of electrons.
(d) Orbital Diagram of the formation of Carbon tetrachloride molecule:

Exercise 4E Very Short Answer Type
4 questionsAnswer:
Property | Atomic number |
---|---|
Element which can form trivalent ion | 15 |
An element with 4 shells | 19 |
Element with 6 valence electrons | 8 |
Element which does not form ion | 10 |
Explanation:
- Electronic configuration of an element with atomic number 15 is [2, 8, 5], hence, it needs three electrons to complete its octet and forms a trivalent ion.
- Electronic configuration of an element with atomic number 19 is [2, 8, 8, 1], hence, it has K, L, M, N shells.
- Electronic configuration of an element with atomic number 8 is [2, 6], hence it has 6 electrons in the outermost shell called valence electrons.
- Electronic configuration of an element with atomic number 10 is [2, 8], so its octet is complete and it does not form ions.
Answer:
(i) Reduction, since there is a gain of 2 electrons by Oxygen.
(ii) Oxidation, since there is a loss of an electron by Potassium.
(iii) Reduction, since there is a gain of an electron by Iron.
(iv) Oxidation, since there is a loss of two electrons by Zinc.
Name or state the following.
(a) An element having valency zero.
(b) Metal with valency one.
(c) Atoms of the same element differing in mass numbers.
(d) Elements having same mass number but different atomic number.
(e) Bond formed by transfer of electrons(s)
(f) Ion formed by gain of electron(s)
Answer:
(a) Helium (Inert element)
(b) Lithium
(c) Isotopes
(d) Isobars
(e) Electrovalent or Ionic bond
(f) Anion