Q: Question 9Draw the geometric atomic structure of each of the following atoms showing the number of electrons, protons and neutrons in each of them :(i) 612C^{12}_6 \text{C}612C(ii) 1123Na^{23}_{11} \text{Na}1123Na(iii) 1531P^{31}_{15}\text{P}1531P(iv) 1939K^{39}_{19} \text{K}1939K(v) 2040Ca^{40}_{20} \text{Ca}2040Ca

(i) Atom of Carbon [612C^{12}_6 \text{C}612C] — No. of Electrons [e] = 6 No. of Protons [p] = 6 No. of Neutrons [n] = A - Z = 12 - 6 = 6 Electronic configuration = 2, 4(ii) Atom of sodium 1123Na^{23}_{11} \text{Na}1123Na — No. of Electrons [e] = 11 No. of Protons [p] = 11 No. of Neutrons [n] = A - Z = 23 - 11 = 12 Electronic configuration = 2, 8, 1(iii) Atom of Phosphorus 1531P^{31}_{15}\text{P}1531P — No. of Electrons [e] = 15 No. of Protons [p] = 15 No. of Neutrons [n] = A - Z = 31 - 15 = 16 Electronic configuration = 2, 8, 5(iv) Atom of Potassium 1939K^{39}_{19} \text{K}1939K — No. of Electrons [e] = 19 No. of Protons [p] = 19 No. of Neutrons [n] = A - Z = 39 - 19 = 20 Electronic configuration = 2, 8, 8, 1(v) Atom of Calcium 2040Ca^{40}_{20} \text{Ca}2040Ca — No. of Electrons [e] = 20 No. of Protons [p] = 20 No. of Neutrons [n] = A - Z = 40 - 20 = 20 Electronic configuration = 2, 8, 8, 2

Q: Question 1(1987)1224Mg^{24}_{12}\text{Mg}1224Mg and 1226Mg^{26}_{12}\text{Mg}1226Mg are symbols of two isotopes of magnesium. Compare the atoms of these isotopes with respect to :(i) the composition of their nuclei.(ii) their electronic configurations.(iii) give the reason why the two isotopes of magnesium have different mass numbers.

(i) The composition of their nuclei :1224Mg^{24}_{12}\text{Mg}1224Mg :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}\text{Mg}1226Mg :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}\text{Mg}1224Mg :As electron [e] = 12 hence, electronic configuration = 2,8,21226Mg^{26}_{12}\text{Mg}1226Mg :As electron [e] = 12 hence, electronic configuration = 2,8,2(iii) 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 1

Question 1State the main postulates of Dalton's atomic theory. How does the modern atomic theory contradict and correlate with Dalton's atomic theory.

Accepted Answer

Postulates of Dalton's Atomic Theory:Matter consist of small indivisible particles called atoms.Atoms are neither created nor destroyed.Atoms of the same element are alike in every aspect and differ from atoms of all other elements.Atoms combine with other atoms in simple whole number ratio's forming compound atoms or molecules.An atom is the smallest unit of matter which takes part in a chemical reaction and all chemical changes result from separation or combination of atoms.Contradictions with Modern atomic theory:Dalton's atomic theoryModern atomic theoryAtoms are indivisible and indestructible.Atoms are divisible and destructible.An atom is the smallest indivisible particle of an elementAtoms are divisible - consisting of subatomic particles like electrons, protons and neutrons.Atoms of the same element are alike in all respects and differ from atoms of other elements.Atoms of the same element may not be alike in all respects as seen in the case of isotopes which are atoms of the same element having same atomic number but different mass numbers.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.

Question 2

Question 2Explain in brief the experimental proofs which led to the discovery of –(i) Electrons(ii) Protons(iii) atomic nucleus(iv) neutrons.

Accepted Answer

(i) Electrons — Experimental proofs leading to the discovery of Electrons were given by William Crookes in 1878 and Sir J.J. Thomson in 1897.William Crookes found that when an electric discharge is passed through a tube containing a gas at low pressure (0.01 mm of Hg), blue rays were emitted from the cathode and were termed cathode rays.Sir J.J. Thomson studied the characteristics and constituents of cathode rays and found that:Cathode rays travel in straight lines from cathode to anode and cast a shadow of the object placed in their path.They are deflected by magnetic and electric field [attracted towards positively charged plate].They have kinetic energy and raise the temperature of a metallic object on which they fall.These findings lead to the conclusion that Cathode rays consist of negatively charged particles now called electrons.(ii) Protons : Goldstein discovered protons.As atoms are electrically neutral there must be equal positively charged particles in atom. This led to discovery of the protons.He used a perforated disc as cathode and found positively charged rays travelling in opposite direction to cathode rays.These rays consisted of positively charged particles which led to the discovery of protons.(iii) Atomic Nucleus : Lord Rutherford discovered atomic nucleus in 1911.He directed alpha particles towards a metallic sheet.The deflection of the alpha particles was observed and concluded that there is a nucleus in the centre of the atom.(iv) Neutrons : James Chadwick discovered neutron in 1932.He established that an atom contains protons and electrons.Electrons have negligible mass, hence the mass is concentrated in the nucleus. ∴ Atomic mass = mass of protons.However, in the case of helium - there are two protons in it's nucleus and hence atomic mass should be equal to twice the mass of each proton i.e., 2 x 1.008 but it's atomic mass is calculated experimentally and found to be 4.003 a.m.u.It was therefore proved that, in the nucleus of an atom, there is other particle called a neutron which has no electrical charge but is almost of an equal mass as the protons.This confirmed the existence of neutrons

Question 3

Question 3State in brief the drawbacks of Rutherford's atomic model correlating them with the postulates of Bohr's atomic model.

Accepted Answer

Drawbacks of Rutherford's Atomic Model :In Rutherford's model of the atom - the electrons revolve around the nucleus and the attractive force of the positively charged nucleus would be balanced by the centrifugal force arising due to the rotation of electron.If an electron moves round the nucleus - it must radiate out energy and gradually move towards the nucleus following a spiral path - till it collides and ultimately collapses in it.Thus, Rutherford's model could not explain correctly the stability of an atom.Postulates of Bohr's Atomic Model :Electrons revolve around the nucleus in fixed orbits or energy levels, possessing certain amount of energy.The integer n represents the various energy levels 1, 2, 3 or K, L, M starting from the innermost.As long as the electrons rotates in one of the energy levels - it neither loses or absorbs energy. When energy is supplied initially to the atoms - an electron moves to an orbit of higher energy. When this electron drops back to the original orbit - energy is radiated by the atom - according to equation E2 - E1 = hγ[E2 and E1 are energy states of the electron in higher and lower orbit.]Bohr's model gave the preciseness of location and mode of rotation of electron around the nucleus.

Question 4

Question 4What is meant by the terms :(i) subatomic particles,(ii) nucleus,(iii) orbits,(iv) atomic number(v) mass number with reference to an atom.

Accepted Answer

(i) Subatomic particles : The particles into which an atom is divided (i.e., protons, electrons, neutrons) are called the subatomic particles.(ii) Nucleus : The centre or the nucleus of the atom is the place where the protons and neutrons are found.(iii) Orbits : The electrons revolve around the nucleus along imaginary paths known as shells or more accurately orbits or orbitals.(iv) Atomic number : Number of protons or number of electrons present in an atom. [Z](v) Mass number [A] : The Mass number of an atom is the total number of protons and neutrons collectively called nucleons in the nucleus of the atom.

Question 5

Question 8The following elements U to Z are given 3U, 6V, 9W, 14X, 18Y, 20ZState the electronic configuration of each and state whether they are metals, non-metals or inert gases.

Accepted Answer

ElementElectronic configurationmetals, non-metals or inert gases3U2,1Metal6V2,4Non-metal [s]9W2,7Non-metal [g]14X2,8,4Metalloid18Y2,8,8Inert gas20Z2,8,8,2Metal

Question 6

Question 9Draw the geometric atomic structure of each of the following atoms showing the number of electrons, protons and neutrons in each of them :(i) 612C^{12}_6 	ext{C}612​C(ii) 1123Na^{23}_{11} 	ext{Na}1123​Na(iii) 1531P^{31}_{15}	ext{P}1531​P(iv) 1939K^{39}_{19} 	ext{K}1939​K(v) 2040Ca^{40}_{20} 	ext{Ca}2040​Ca

Accepted Answer

(i) Atom of Carbon [612C^{12}_6 	ext{C}612​C] —    No. of Electrons [e] = 6    No. of Protons [p] = 6    No. of Neutrons [n] = A - Z    = 12 - 6 = 6    Electronic configuration = 2, 4(ii) Atom of sodium 1123Na^{23}_{11} 	ext{Na}1123​Na —    No. of Electrons [e] = 11    No. of Protons [p] = 11    No. of Neutrons [n] = A - Z    = 23 - 11 = 12    Electronic configuration = 2, 8, 1(iii) Atom of Phosphorus 1531P^{31}_{15}	ext{P}1531​P —     No. of Electrons [e] = 15     No. of Protons [p] = 15     No. of Neutrons [n] = A - Z     = 31 - 15 = 16     Electronic configuration = 2, 8, 5(iv) Atom of Potassium 1939K^{39}_{19} 	ext{K}1939​K —     No. of Electrons [e] = 19     No. of Protons [p] = 19     No. of Neutrons [n] = A - Z     = 39 - 19 = 20     Electronic configuration = 2, 8, 8, 1(v) Atom of Calcium 2040Ca^{40}_{20} 	ext{Ca}2040​Ca —     No. of Electrons [e] = 20     No. of Protons [p] = 20     No. of Neutrons [n] = A - Z     = 40 - 20 = 20     Electronic configuration = 2, 8, 8, 2

Question 7

Question 10Define an 'isotope'. Give reasons why isotopes have same chemical but different physical properties.

Accepted Answer

Isotopes are atoms of the same element having same atomic number but different mass numbers.Chemical properties of isotopes of the same element are similar because chemical properties are dependent on the electronic configuration of an atom. As Isotopes have same atomic number so they have same number of electrons and hence same electronic configuration.Physical properties of isotopes of the same element are different because physical properties are dependent on the atomic mass. Isotopes have different mass number [A], different number of neutrons and hence different atomic masses giving them different physical properties.

Question 8

Question 11Draw the geometric atomic structure of the three isotopes of hydrogen and the two isotopes of chlorine.

Accepted Answer

Geometric atomic structures of three isotopes of hydrogen is shown in the diagram below:Geometric atomic structures of two isotopes of chlorine is shown in the diagram below:

Question 9

Question 15.2Differentiate between the terms —Duplet and octet rule.

Accepted Answer

Duplet RuleOctet RuleFor an atom to achieve stable electronic configuration it must have 2 electrons in the first shell [outermost] like that of noble gas Helium.For an atom to achieve stable electronic configuration it must have 8 electrons in outermost orbit like that of noble gases other than Helium.

Question 10

Question 16Explain the octet rule for formation of —(a) Sodium chloride from a sodium atom and a chlorine atom.(b) Nitrogen molecule from two nitrogen atoms.

Accepted Answer

(a) As Na atom has atomic number 11 and electronic configuration [2, 8, 1]. Therefore, it tends to lose 1 electron from its valence shell forming Na1+ (cation) to attain a stable octet configuration of the nearest noble gas Neon [2, 8].Cl, on the other hand has atomic number 17 and electronic configuration [2, 8, 7]. Therefore, it tries to gain one electron in its valence shell forming Cl1- (anion) to attain a stable octet configuration of the nearest noble gas Argon [2, 8, 8].The oppositely charged cation and anion attract each other with a force - [electrostatic force of attraction] resulting in formation of an ionic or electrovalent compound [NaCl] as shown below.(b) Nitrogen atom (147N) has atomic number 7 and electronic configuration [2, 5]. To attain stable octet structure of the nearest noble gas Neon [2, 8], Nitrogen needs three electrons in the valence shell. Nitrogen atom shares electron pairs with other nitrogen atom resulting in the formation of a molecular or covalent bond as shown below:

Question 11

Question 1(1987)1224Mg^{24}_{12}	ext{Mg}1224​Mg and 1226Mg^{26}_{12}	ext{Mg}1226​Mg are symbols of two isotopes of magnesium. Compare the atoms of these isotopes with respect to :(i) the composition of their nuclei.(ii) their electronic configurations.(iii) give the reason why the two isotopes of magnesium have different mass numbers.

Accepted Answer

(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(iii) 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 12

Question 2(1987)Chlorine is an element of atomic number 17. It is a mixture of two isotopes having mass number of 35 and 37.(i) What is meant by 'atomic number of an element' ? What do you understand by an 'atom'.(ii) Write down the electronic configuration of the chlorine atom.(iii) State the number of protons, electrons and neutrons in the following isotopes: 35Cl17^{35}	ext{Cl}_{17}35Cl17​, 37Cl17^{37}	ext{Cl}_{17}37Cl17​(iv) Explain why the two atoms in (iii) above have the same chemical reactions.(v) If molten magnesium chloride is electrolyzed suggest a suitable electrode [anode].

Accepted Answer

(i) Atomic number [Z] of an element is the number of :Protons - in the nucleus of an atomElectrons - in the complete atomPositive charges - in the nucleus of it's atomAn atom is the smallest particle of an element which can take part in a chemical reaction and is considered the basic unit of matter. e.g. O,Cl are atoms.(ii) Atomic number of Chlorine is 17 so it has 17 electrons. Hence, its electronic configuration = 2, 8, 7(iii) 35Cl17^{35}	ext{Cl}_{17}35Cl17​ —No. of protons = 17No. of electrons = 17No. of neutrons = A - Z = 35 - 17 = 1837Cl17^{37}	ext{Cl}_{17}37Cl17​ —No. of protons = 17No. of electrons = 17No. of neutrons = A - Z = 37 - 17 = 20(iv) As the two atoms in (iii) above are isotopes and hence have same electronic configuration. Therefore, they have similar chemical properties because chemical properties are dependent on electronic configuration of an element.(iv) Carbon anode is used when molten magnesium chloride is electrolyzed.

Question 13

Question 1(1988)Five atoms are labelled V to ZAtomsMass NumberAtomic numberV4020W199X73Y168Z147(i) Which one of these atoms (1) contains 7 protons; (2) has an electronic configuration 2, 7 ?(ii) Write down the formula of the compound formed by atoms X and Y.

Accepted Answer

(i) As Z has atomic number of 7, hence it has 7 protons. Electronic configuration of 2, 7 means the atom has 2 + 7 = 9 electrons. Therefore, element W has electronic configuration 2, 7 since its atomic number is 9.(ii) Number of electron in X is 3. Therefore, electronic configuration = (2, 1). As X will try to lose 1 electron to attain stable state, hence, X has valency [1+]Number of electron in Y is 8. Therefore, electronic configuration = (2, 6). As Y will try to gain 2 electrons to attain stable state, hence, Y has valency [2-]Since valency of X is 1+ and valency of Y is 2-Formula of the compound:X1+↗Y2−X11 ↘↙ Y2⇒X22 ↘↙ Y1\text{X}^{1+} \phantom{\nearrow} \text{Y}^{2-} \[0.5em] \overset{\phantom{1}{1}}{\text{X}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{Y}} \Rightarrow \underset{\phantom{2}{2}}{\text{X}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{Y}} \[0.5em]X1+↗Y2−X11 ↘↙ Y2⇒22X​ ↘↙ 1Y​So, we get the formula as X2Y\bold{X}_2\bold{Y}X2​Y

Question 14

Question 3(1990)Elements A, B and C have atomic numbers 9, 20 and 10 respectively.(i) State which one is (1) a non-metal; (2) a metal; (3) chemically inert.(ii) Write down the formula of the compound formed by two of the above elements.

Accepted Answer

(i) The electronic configurations of the elements are:    A[9] = [2, 7]    B[20] = [2, 8, 8, 2]    C[10] = [2, 8](1) A is a non-metal as it will gain electron to attain a stable octet.(2) B is a metal as it will lose two electrons to attain a stable octet.(3) C is chemically inert as it's valence shell is completely filled.(ii) A has 7 electrons in the outermost shell. Therefore, it will try to gain one electron to attain stable octet, hence, A has valency [1-]As B has 2 in the outermost shell. Therefore, it will try to lose 2 electrons to attain stable octet, hence, B has valency [2+]Since valency of A is 1- and valency of B is 2+Formula of the compound:B2+↗A1−B22 ↘↙ A1⇒B11 ↘↙ A2\text{B}^{2+} \phantom{\nearrow} \text{A}^{1-} \[0.5em] \overset{\phantom{2}{2}}{\text{B}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{A}} \Rightarrow \underset{\phantom{1}{1}}{\text{B}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{A}} \[0.5em]B2+↗A1−B22 ↘↙ A1⇒11B​ ↘↙ 2A​So, we get the formula as BA2\bold{BA}_\bold{2}BA2​

Question 15

Question 4(1990)Define : Proton, Electron, Neutron.

Accepted Answer

Proton [p] is a subatomic particle in the nucleus of an atom having unit positive charge 1.602 x 10-19 coulombs and mass of 1.670 x 10-24 g. It is 1837 times as heavy as an electron.Electron [e] is a subatomic particle revolving around the nucleus of atom having unit negative charge 1.602 x 10-19 coulombs, and mass of 9.107 x 10-28 g. It is 1⁄1837 of the mass of a hydrogen atom or proton.Neutrons [n] is a subatomic particle found in the nucleus of an atom, carrying no electric charge and mass equal to the mass of proton i.e., 1.676 x 10-24 g.

Question 16

Question 1(1991)Write down the electronic configuration of the following:(i) 1327X^{27}_{13}	ext{X}1327​X,(ii) 1735Y^{35}_{17}	ext{Y}1735​Y.Write down the number of electrons in X and neutrons in Y and the formula of the compound formed by X and Y.

Accepted Answer

(i) 1327X^{27}_{13}\text{X}1327​X = [2, 8, 3] ; Number of electrons in X = 13(ii) 1735Y^{35}_{17}\text{Y}1735​Y = [2, 8, 7] ; Number of neutrons = A - Z = 35 - 17 = 18Valency of 1327X^{27}_{13}\text{X}1327​X = [2, 8, 3] = 3 positiveValency of 1735Y^{35}_{17}\text{Y}1735​Y = [2, 8, 7] = 1 negative.Formula of the compound:X3+↗Y1−X33 ↘↙ Y1⇒X11 ↘↙ Y3\text{X}^{3+} \phantom{\nearrow} \text{Y}^{1-} \[0.5em] \overset{\phantom{3}{3}}{\text{X}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{Y}} \Rightarrow \underset{\phantom{1}{1}}{\text{X}} \space {\searrow}\mathllap{\swarrow} \space \underset{3}{\text{Y}} \[0.5em]X3+↗Y1−X33 ↘↙ Y1⇒11X​ ↘↙ 3Y​So, we get the formula as XY3\bold{XY}_\bold{3}XY3​

Question 17

Question 1(1992)According to the Dalton's Atomic Theory, atoms of the same element are identical in all respects. But according to the Modern Atomic Theory, this postulate is proved wrong. Explain.

Accepted Answer

According to modern atomic theory atoms of the same element may not be alike in all respect as seen in case of isotopes which are atoms of the same element having same atomic number but different mass numbers.e.g. isotopes of chlorine 1735Cl^{35}_{17}	ext{Cl}1735​Cl and 1737Cl^{37}_{17}	ext{Cl}1737​Cl have different atomic masses 35 and 37. This proves Dalton's Atomic Theory wrong.

Question 18

Question 1(1994)What is a proton ? What is the significance of the number of protons found in the atoms of each of the different elements.

Accepted Answer

Proton [p] is a subatomic particle in the nucleus of an atom having unit positive charge 1.602 x 10-19 coulombs and mass of 1.670 x 10-24 g. It is 1837 times as heavy as an electron.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 behaviour of an atom. Protons also determine the positive charge of the nucleus in the atom.

Question 19

Question 2(1994)What is the relation between the number of protons and the number of electrons in an atom.

Accepted Answer

The number of protons in an atom is equal to the number of electrons.

Question 20

Question 3(1994)What would be the reason for an element to have atoms with differing mass numbers.

Accepted Answer

Atoms of same element have different mass number due to the difference in number of neutrons.

Atomic Structure

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