Question 1

Question 11On the basis of Thomson's model of an atom, explain how an atom as a whole is neutral.

Accepted 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.

Question 2

Question 14(a) Which metal did Rutherford select for his α-particle scattering experiment and why ?(b) What do you think would be the observation of α-particle scattering experiment if carried out on(i) heavy nucleus like platinum(ii) light nuclei like lithium.

Accepted 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.

Question 3

Question 20Give the postulates of Bohr's atomic model.

Accepted Answer

Postulates of Bohr's atomic model are as followsThe 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.

Question 4

Question 5Write down the names of the particles represented by the following symbols and explain the meaning of superscript and subscript numbers attached.1p1, 0n1, -1e0

Accepted Answer

p = protonn = neutrone = electronParticleSymbolSuperscriptCharge(electronic unit)Subscriptmass(amu)Electron-1e0-111840\dfrac{1}{1840}18401​ = 0Proton1p1+11Neutron0n101

Question 5

Question 7Draw the orbital diagram of 2040Ca+2 ion and state the number of three fundamental particles present in it.

Accepted Answer

Below is the orbital diagram of 2040Ca+2 ion:Three fundamental particles of 2040Ca+2 are:Protons — 20Electrons — 18Neutrons — 40 - 20 = 20

Question 6

Question 12Identify the most stable atom from the following list. Give reason for your answer2311Na^{23}{11} ext{Na}2311Na, 3517Cl^{35}{17} ext{Cl}3517Cl, 4018Ar^{40}{18} ext{Ar}4018Ar, 2713Al^{27}{13} ext{Al}2713Al

Accepted Answer

The most stable atom from the given list is 1840Ar^{40}_{18} ext{Ar}1840Ar (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, 2311Na^{23}{11} ext{Na}2311Na, 3517Cl^{35}{17} ext{Cl}3517Cl, 1327Al^{27}_{13} ext{Al}1327Al, have incomplete outer shells and are therefore reactive as they tend to gain, lose, or share electrons to achieve stability.

Question 7

Question 15Sulphur has an atomic number 16 and mass number of 32. State the number of protons and neutrons in the nucleus of sulphur. Give a simple diagram to show the arrangement of electrons in an atom of sulphur.

Accepted Answer

Given, Atomic number = 16 and mass number = 32So, Number of protons = 16Number of electrons = 16Number of neutrons = 32 – 16 = 16Electronic configuration = 2, 8, 6Below diagram shows the arrangement of the electrons in an atom of sulphur:

Question 8

Question 16Write down the electronic configuration of the following:(a) 1327X(b) 1735YWrite down the number of electrons in X and neutrons in Y.

Accepted Answer

(a) electronic configuration of 1327X is [2, 8, 3](b) electronic configuration of 1735Y is [2, 8, 7]Number of electrons in X = 13Mass number of Y = 35 and number of protons in Y = 17∴ Number of neutrons in Y = 35 - 17 = 18

Question 9

Question 3Match the atomic numbers 4, 14, 8, 15 and 19 with each of the following :(a) A solid non-metal of valency 3.(b) A gas of valency 2.(c) A metal of valency 1.(d) A non-metal of valency 4.

Accepted Answer

PropertyAtomic NumberA solid non-metal of valency 315A gas of valency 28A metal of valency 119A non-metal of valency 414Explanation(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.

Question 10

Question 4(a) What are inert elements?(b) Why do they exist as monoatoms in molecules?(c) What are valence electrons?

Accepted 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.

Question 11

Question 5In what respect do the three isotopes of hydrogen differ ? Give their structures.

Accepted 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

Question 12

Question 8Define the terms :(a) mass number(b) ion(c) cation(d) anion(e) element(f) orbit.

Accepted 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 neutronsMass 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.

Question 13

Question 11Five atoms are labelled A to E.AtomsMass no.Atomic no.A4020B199C73D168E147(a) Which one of these atoms:(i) contains 7 protons(ii) has electronic configuration 2, 7?(b) Write down the formula of the compound formed between C and D.(c) Predict :(i) metals(ii) non-metals.

Accepted 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.

Question 14

Question 121224Mg^{24}_{12}	ext{Mg}1224​Mg and 1226Mg^{26}_{12}	ext{Mg}1226​Mg are symbols of two isotopes of magnesium.(a) Compare the atoms of these isotopes with respect to :     (i) the composition of their nuclei.     (ii) their electronic configurations.(b) Give reasons why the two isotopes of magnesium have different mass numbers.

Accepted Answer

(a) (i) The composition of their nuclei :1224Mg^{24}_{12}	ext{Mg}1224​Mg :Atomic number [Z] = electron [e] = proton [p] = 12Mass number [A] = 24As, neutron [n] = A - Z = 24 - 12 = 12.Hence, p = 12, n = 121226Mg^{26}_{12}	ext{Mg}1226​Mg :Atomic number [Z] = electron [e] = proton [p] = 12Mass number [A] = 26As, neutron [n] = A - Z = 26 - 12 = 14.Hence, p = 12, n = 14(ii) Their electronic configurations :1224Mg^{24}_{12}	ext{Mg}1224​Mg :As electron [e] = 12 hence, electronic configuration = 2,8,21226Mg^{26}_{12}	ext{Mg}1226​Mg :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.

Question 15

Question 13What are nucleons? How many nucleons are present in phosphorus? Draw its structure.

Accepted 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 = 16Therefore, number of nucleons are p + n = 15 + 16 = 31.Structure of Phosphorus is shown in the diagram below:

Question 16

Question 14What are isotopes? With reference to which fundamental particle do isotopes differ ? Give two uses of isotopes.

Accepted Answer

Isotopes are atoms of the same element having same atomic number but different mass number.For example : 2412Mg and 2612MgIsotopes differ in the number of neutrons.Uses of isotopesRadioactive 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.

Question 17

Question 15Why do 3517Cl and 3717Cl have the same chemical properties? In what respect do these atoms differ?

Accepted 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.

Question 18

Question 16Explain fractional atomic mass. What is the fractional mass of chlorine?

Accepted 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(35×3)+(37×1)4\dfrac{(35	imes3) + (37	imes1)}{4}4(35×3)+(37×1)​ = 35.5. Therefore, atomic mass of chlorine atom is 35.5

Question 19

Question 17(a) What is meant by 'atomic number of an element'?(b) Complete the table given below:  No of protonsNo of electronsNo of neutronsAtomic numberMass number3517Cl     3717Cl     (c) Write down the electronic configuration of(i) Chlorine atom(ii) Chloride ion

Accepted 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 protonsNo of electronsNo of neutronsAtomic numberMass number3517Cl17171817353717Cl1717201737(c) Write down the electronic configuration of(i) Chlorine atom — 2, 8, 7(ii) Chloride ion — 2, 8, 8

Question 20

Question 18Draw diagrams representing the atomic structures of the following(a) Sodium atom(b) Chloride ion(c) Carbon atom(d) Oxide ion.

Accepted Answer

(a) Sodium atom —(b) Chloride ion —(c) Carbon atom —(d) Oxide ion —

Atomic Structure and Chemical Bonding

Exercise 4A

Exercise 4B

Exercise 4C

Exercise 4D

Exercise 4E Descriptive Type

Exercise 4E Multiple Choice Type

Exercise 4E Numericals

Exercise 4E Short Answer Type

Exercise 4E Structuredapplicationskill Type