CHAPTER 02

STRUCTURE OF
ATOM
Chemistry Smart Booklet
Theory + NCERT
MCQs+Topicwise Practice
MCQs + NEET PYQs

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 STRUCTURE OF ATOM
Introduction:
The word "atom" has been derived from the Greek word 'atoms’ which mans 'indivisible’.
These early ideas were mere speculation and there was no way to test them experimentally.

Atomic Structure:
Atom is made up of smaller units like proton, neutron and electron. Some other particles
like positron, neutrino, antineutrino, π-meson, μ-meson, k meson etc are also present
which are very short lived.

Discovery of Electron
In 1879, William Crooks studied the conduction of electricity through gases at low
pressure. He performed the experiment in a discharge tube which is a cylindrical hard glass
tube about 60 cm in length. It is sealed at both the ends and fitted with two metal electrodes.
The electrical discharge through the gases could be observed only at very low pressures
and at very high voltages.

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J.J. Thomson took a discharge tube and applied a voltage of a 10000 volt potential difference
across it at a pressure of 10–2 mm of Hg. He found some glowing behind anode. It means
some invisible rays produced at cathode strike behind anode and produce fluorescence. He
named them cathode rays.

Properties of Cathode Rays

i. These rays have mechanical energy and travel in straight line.
ii. These rays are deflected towards positive plate of electric field. It means these are
made up of negatively charged particle called electron.
iii. Colour observed is independent from nature of gas.
iv. Mulliken determined the charge on electron which is 1.602 × 10-19C.
v. Specific charge on electron is calculated by J.J. Thomson.

Charge to mass ratio

J.J. Thomson for the first time experimentally determined charge/mass ratio called e/m
ratio for the electrons. For this, he subjected the beam of electrons released in the discharge
tube as cathode rays to influence the electric and magnetic fields. These were acting
perpendicular to one another as well as to the path followed by electrons.
According to Thomson, the amount of deviation of the particles from their path in
presence of electrical and magnetic field depends on,
1. Magnitude of the negative charge on particle
2. Mass of particle
3. Strength of magnetic field

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 When electric field is applied, deviation from path takes place. If only electric field is
applied, cathode rays strike at A. If only magnetic field is applied, cathode rays strike at
C. In absence of any field, cathode rays strike at B.
By carrying out accurate measurements on the amount of deflections observed by the
electrons on the electric field strength or magnetic field strength, Thomson was able to

determine the value of e/me = 1.758820 x 1011 C kg-1

where me = Mass of the electron in kg

e = magnitude of charge on the electron in coulomb (C).

Discovery of anode rays

In 1886, Goldstein modified the discharge tube by using a perforated cathode. On
reducing the pressure, he observed a new type of luminous rays passing through the holes
or perforations of the cathode and moving in a direction opposite to the cathode rays.
These rays were named as positive rays or anode rays or as canal rays. Anode rays are not
emitted from the anode but from a space between anode and cathode.

Properties of anode rays

1. These rays deflect towards negative plate of applied electric field. It means these are
made up of positively charged particle.
2. Property of anode rays depends on nature of gas.
3. These rays travel in straight line and have mechanical energy.

Discovery of Neutron
Chadwick in 1932 found the evidence for the production of neutron in given reaction.
4Be9 + 2He4 ⟶ 6C12 + 0n1

Neutron is chargeless particle and have mass equal to proton.

Millikan’s Oil Drop Experiment

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 In this experiment, some fine oil droplets were allowed to enter through a tiny hole into
the upper plate of electrical condenser. These oil droplets were produced by atomiser. The
air in the chamber was subjected to the ionization by X-rays. The electrons produced by
the ionization of air attach themselves to the oil drops.

Thus oil droplets acquire negative charge. When sufficient amount of electric field is
applied, the motion of the droplets can be accelerated, retarded or made stationary.
Millikan observed that the smallest charge found on them was –1.6 × 10–19 coulomb and
the magnitude of electrical charge, q on the droplets is always an integral multiple of the
electrical charge ‘e’ i.e., q = ne

Thomson’s Model of Atom

J.J. Thomson in 1898, proposed a model of atom which looked more or less like plum
pudding or raisin pudding. He assumed atom to be a spherical body in which electrons
are unevenly distributed in a sphere having positive charge which balance the electron’s
charge. It is called Plum pudding model.

Important Feature of This Model: The mass of the atom is assumed to be uniformly
distributed over whole atom.

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 Failure: This model was able to explain the overall neutrality of the atom, it could not
satisfactorily, explain the results of scattering experiments carried out by Rutherford in
1911.

Rutherford's Model
Rutherford in 1911, performed some scattering experiments in which he bombarded thin
foils of metals like gold, silver, platinum or copper with a beam of fast moving a-particles.
The thin gold foil had a circular fluorescent zinc sulphide screen around it. Whenever a-
particles struck the screen, a tiny flash of light was produced at that point.

From these experiments, he made the following observations:

1. Most of the α-particles pass without any deviation.
2. Few particles deviate with small angle.
3. Rare particles retrace its path or show deflection greater than 90°.

On the basis of these observation, he proposed a model.

1. Atom is of spherical shape having size of order 10–10 meters.
2. Whole mass is concentrated in centre called nucleus having size of order 10–15
meters.

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 3. Electron revolves around the nucleus in circular path like planets revolve around
sun.
Limitation: This model could not explain stability of atom. According to Maxwell's classic
theory, an accelerated charged particle liberates energy. So, during revolution, it must
radiate energy and by following the spiral path it should comes on nucleus.

Atomic number
It is equal to the number of protons present in the nucleus of an atom. Atomic number is
designated by the letter ‘Z’. In case of neutral atom atomic number is equal to the number
of protons and even equal to the number of electrons in atom.
Z = Number of protons (p) = Number of electrons (e)

Mass number
It is equal to the sum of the positively charged protons (p) and electrically neutral neutrons
(n). Mass number of an atom is designated by the letter ‘A’.
Mass number (A) = Number of protons (p or Z) + Number of neutrons (n)
Note: The atom of an element X having mass number (A) and atomic number (Z) may be
represented by a symbol ZXA.

Isotopes
Atoms with identical atomic number but different atomic mass number are known as
Isotopes. Isotopes of Hydrogen 1H1, 1H2 and 1H3

Isobars
Isobars are the atom with the same mass number but different atomic number, for example
6C14 and 7N14

Electromagnetic Waves Theory

This theory was put forward by James Clark Maxwell in 1864. Electromagnetic Waves
are the waves which are produced by varying electric field and magnetic field which are
perpendicular to each other in the direction perpendicular to both of them.

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 The main points of this theory are as follows:
1. The energy is emitted from any source continuously in the form of radiations and
is called the radiant energy.
2. The radiations consist of electric and magnetic fields oscillating perpendicular to
each other and both perpendicular to the direction of propagation of the radiation.
3. The radiations possess wave character and travel with the velocity of light 3 × 108
m/sec.
4. These waves do not require any material medium for propagation. For example,
rays from the sun reach us through space which is a non-material medium.

Characteristics of a Wave
Wavelength (λ): It is the distance between two consecutive crests or troughs and is
denoted by λ.
Frequency (v): It is the number of waves passing through a given point in one second. The
unit frequency is hertz or cycle per second.
Wave number: It is the number of waves in a unit cycle. wave number =1λ=1λ
Velocity: Velocity of a wave is defined as the linear distance travelled by the wave in one
second. It is represented by c and is expressed in m/sec.
Amplitude: Amplitude of a wave is the height of the crest or the depth of the through. It
is represented by V and is expressed in the units of length.

Black Body Radiations

Black-body is an ideal body which emits and absorbs radiations of all frequencies. The
radiation emitted by these bodies is called black-body radiation.

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 At a given temperature, the intensity and frequency of the emitted radiation depends is
temperature. At a given temperature, the intensity of radiation emitted increases with
decrease of wavelength.

Photoelectric Effect
When light of a suitable frequency is allowed to incident on a metal, ejection of electrons
take place. This phenomenon is known as photo electric effect.

Observations in Photoelectric Effect

1. Only photons of light of certain minimum frequency called threshold frequency (v0)
can cause the photoelectric effect. The value of v0 is different for different metals.
2. The kinetic energy of the electrons which are emitted is directly proportional to the
frequency of the striking photons and is quite independent of their intensity.
3. The number of electrons that are ejected per second from the metal surface depends
upon the intensity of the striking photons or radiations and not upon their frequency.

Explanation of Photoelectric Effect

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 Einstein in (1905) was able to give an explanation of the different points of the
photoelectric effect using Planck’s quantum theory as under:
1. Photoelectrons are ejected only when the incident light has a certain minimum
frequency (threshold frequency v0).
2. If the frequency of the incident light (v) is more than the threshold frequency (v 0),
the excess energy (hv–hv0) is imparted to the electron as kinetic energy.
3. On increasing the intensity of light, more electrons are ejected but the energies of the
electrons are not altered.
K.E. of the ejected electron.
1
mv2 = hv – hv0
2

Planck's Theory
According to this theory, energy cannot be absorbed or released continuously but it is
emitted or released in the form of small packets called quanta. In case of light this quanta
is known as photon. This photon travels with speed of light. Energy of the photon is
directly proportional to frequency.

E∝ν
E = hν
h is Planck's constant, value is 6.62 × 10–34 Js

Bohr’s Model
1. Niels Bohr in 1913, proposed a new model of atom on the basis of Planck’s Quantum
Theory. The main points of this model are as follows:
2. Atom is of spherical shape having size (of order 10–10 metre).
3. Whole mass is concentrated in centre called nucleus (having order of size 10–15 metre).
4. Electron revolves around nucleus only in limited circular path and he assumed that
electron does not radiate energy during its revolution in permitted paths.
5. Only those orbits are allowed whose orbit angular momentum is integral multiple
of h2πh2π.
6. mvr = nh/2π, where n = 1, 2, 3, 4...
7. When electron absorbs energy, it jumps to higher orbit and when it comes back, it
radiates energy. This postulate explain spectra.

Achievements of Bohr’s Theory

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 1. Bohr’s theory has explained the stability of an atom.
2. Bohr’s theory has helped in calculating the energy of electron in hydrogen atom and
one electron species.
3. Bohr’s theory has explained the atomic spectrum of hydrogen atom.

Limitations of Bohr’s Model

1. The theory could not explain the atomic spectra of the atoms containing more than
one electron or multielectron atoms.
2. Bohr's theory failed to explain the fine structure of the spectral lines.
3. Bohr’s theory could not offer any satisfactory explanation of Zeeman effect and Stark
effect.
4. Bohr’s theory failed to explain the ability of atoms to form molecule formed by
chemical bonds.
5. It was not in accordance with the Heisenberg’s uncertainty principle.

Spectra
The most compelling evidence for the quantization of energy comes from spectroscopy.
Spectrum word is taken from Latin word which means appearance. The record of the
intensity transmitted or scattered by a molecule as a function of frequency or wavelength
is called its spectrum.

Cosmic rays < gamma rays < x rays < ultraviolet rays < visible rays < infra red < micro
waves < radio waves.
Line Spectrum of Hydrogen Atom

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When electric discharge is passed through hydrogen gas enclosed in discharge tube under
low pressure and the emitted light is analysed by a spectroscope, the spectrum consists of
a large number of lines which are grouped into different series. The complete spectrum is
known as hydrogen spectrum.
On the basis of experimental observations, Johannes Rydberg noted that all series of lines
in the hydrogen spectrum could be described by the following expression:
wave number = 1λ = R(1n21 − 1n22)1λ = R(1n12 - 1n22)
R = Rydberg constant
R = 109678 cm–1

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Zeeman Effect
When spectral line (source) is placed in magnetic field, spectral lines split up into sublines.
This is known as zeeman effect.

Stark Effect
If splitting of spectral lines take place in electric field, then it is known as stark effect.

Dual Behaviour of Matter (de Broglie Equation)

De Broglie in 1924, proposed that matter, like radiation, should also exhibit dual behaviour
i.e., both particle like and wave like properties. This means that like photons, electrons also
have momentum as well as wavelength.
Assume light have wave nature, then its energy should be given by Planck's theory
E = hνE = hν …(i)
If it have particle nature, then its energy should be given by Einstein relation,
E = mc2 …(ii)
On comparing equation (i) and (ii),

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 hν = mc2
λ = hmc (for light) …(iii)
For other matter,
λ = hmv …(iv)
λ = hp …(v)
where p = momentum
This equation is called de Broglie equation.

Heisenberg’s Uncertainty Principle

It states that, "It is impossible to measure simultaneously the exact position and exact
momentum of a microscopic particle".
If uncertainty in position = Δ × and
Uncertainty in momentum = ΔP
When both are measured simultaneously, According to this principle,
Δ ×.ΔP ≥ h4π

Quantum Numbers
There are a set of four quantum numbers which specify the energy, size, shape and
orientation of an orbital. To specify an orbital only three quantum numbers are required
while to specify an electron all four quantum numbers are required.
1. Principal quantum number (n): It identifies shell, determines sizes and energy of
orbitals. It is indicated by ‘n’ and its values are 1, 2, 3, 4...
2. Azimuthal quantum number (l): Azimuthal quantum number. ‘l’ is also known as
orbital angular momentum or subsidiary quantum number. l. It identifies sub-shell,
determines the shape of orbitals, energy of orbitals in multi-electron atoms along with
principal quantum number and orbital angular momentum, i.e., The number of
orbitals in a sub shell = 2l + 1. For a given value of n, it can have n values ranging from
0 to n-1.
3. Magnetic quantum number (ml): It gives information about the spatial orientation of
the orbital with respect to standard set of co-ordinate axis.For any sub-shell (defined
by ‘l’ value) 2l+1 values of ml are possible. For each value of l, ml = – l, – (l–1), – (l–2)...
0,1...(l–2), (l–1), l
4. Electron spin quantum number (ms): It refers to orientation of the spin of the electron.
It can have two values +1/2 and -1/2. +1/2 identifies the clockwise spin and -1/2

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 identifies the anti-clockwise spin.

Shape of Atomic Orbitals

Shapes of s-orbitals: s-orbital is present in the s-sub shell. For this sub shell, l = 0 and ml
= 0. Thus, s-orbital with only one orientation has a spherical shape with uniform electron
density along all the three axes. The probability of Is electron is found to be maximum near
the nucleus and decreases with the increase in the distance from the nucleus.

Shapes of p-orbitals: p-orbitals are present in the p-subshell for which l = 1 and ml can
have three possible orientations –1, 0, +1. Thus, there are three orbitals in the p-subshell
which are designated as px, py and pz orbitals depending upon the axis along which they
are directed. The general shape of a p-orbital is dumb-bell consisting of two portions
known as lobes.

Shapes of d-orbitals: d-orbitals are present in d-subshell for which l = 2 and ml = -2, -1, 0,
+1 and +2. This means that there are five orientations leading to five different orbitals. d
orbitals are of five types: dxy, dyz, dzx, dx2-y2, dz2

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Electronic Configuration
Distribution of electron in various orbitals is known as electronic configuration. The
electrons filled in orbitals must obey the following rules-
 Aufbau’s principle
 Pauli’s exclusion principle
 Hund’s rule of maximum multiplicity
1. Aufbau’s principle: According to this principle, orbitals with lowest energy are filled
before the orbitals having higher energy.

1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s < 5f < 6d
< 7p

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 (n + l) rule (Bohr Bury’s Rule)
According to this, The orbital which has lower value of (n + l) is lower in energy.
2. Pauli’s exclusion principle: According to this principle, in an atom, no two electrons
have same value of all the four quantum numbers. In the same orbital, electron always
accommodate in opposite spins. An orbital can have a maximum of two electrons, with
opposite spin.
3. Hund’s rule of maximum multiplicity: According to this rule, electrons are distributed
among the orbital of a subshell in such a way so as to give the maximum number of
unpaired electrons with a parallel spin.

Summary-
1. Atomic number: It is equal to the number of protons in the nucleus of an atom.
2. Mass number: It is equal to the sum of the positively charged protons (p) and
electrically neutral neutrons (n).
3. Isotopes: Isotopes are the atoms of the same element which have the same atomic
number but different mass numbers.
4. Isobars: Isobars are the atoms of different elements having the same mass number but
different atomic numbers.
5. Isoelectronic species: These are those species which have same number of electrons.
6. Radiations: These are defined as the emission or transmission of energy through space
in the form of waves.
7. Electromagnetic waves: The waves which consist of oscillating electric and magnetic
fields are called electromagnetic waves.

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8. Electromagnetic radiations: Those radiations which are associated with electric and
magnetic field are called electromagnetic radiations.
9. Electromagnetic spectrum: The arrangement of the various types of electromagnetic
radiations in the order of increasing or decreasing wavelengths or frequencies is
known as electromagnetic spectrum.
10. Wavelength (λ): It is the distance between successive points of equal phase of a wave.
11. Frequency (f): The number of waves that pass a given point in one second is known
as the frequency.
12. Time period (T): Time taken by the wave for one complete cycle or vibration is called
time period.
13. Velocity (v): It is the distance travelled by a wave in one second.
14. Wave number: It is defined as the number of wavelengths per unit length.
15. Threshold frequency: It is the minimum frequency of light needed to cause the
photoelectric effect.
16. Continuous spectrum: The combination of light of different frequencies in continuous
manner is called continuous spectrum.
17. Line spectrum: The spectrum of atoms consist of sharp well-defined lines
corresponding to definite frequencies is called line spectrum.
18. Spectroscopy: The study of emission or absorption spectra is called spectroscopy.
19. Quantization: The restriction of a property to discrete values and not continuous
values is called quantization.
20. Quantum mechanics: The branch of science that takes into account the dual behaviour
of matter is called quantum mechanics.
21. Atomic orbital: It is the region of space where the probability of finding the electron
is maximum.
22. Quantum numbers: may be defined as a set of four numbers with the help of which
we can get complete information about electron in an atom.

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 NCERT LINE BY LINE QUESTIONS
(1.) The measurement of the electron position is associated with an uncertainty in momentum of
5 1020 gcms-1 . The uncertainty in electron velocity is [Page: 52]
(a.) 5.6 107 cm / s (b.) 5.6 107 m / s

(c.) 6.5 107 cm / s (d.) 6.5 107 m / s

(2.) Match the following: [NCERT Exemplar Modified, Page: 38]
 i. X ‐ Rays  P.   1010 Hz
 ii. Long radio waves  Q.   1022 Hz
 iii. Microwaves  R.   100  104 Hz
 iv.   Rays S.   1018 Hz
(a.) (i)‐S, (ii)‐P, (iii)‐R, (iv)‐Q (b.) (i)‐R, (ii)‐Q, (iii)‐P, (iv)‐S

(c.) (i)‐S, (ii)‐ R, (iii)‐ P, (iv)‐Q (d.) (i)‐S, (ii)‐ Q, (iii)‐ P, (iv)‐R

(3.) For the electrons of oxygen atom, which of the following statements is incorrect? [NCERT Exemplar
Modified, Page: 6l]
(a.) Zeff for an electron in a 2s‐orbital is greater (b.) An electron in the 2s‐orbital has lesser energy
than the Zeff for an electron in a 2p‐orbital. as an electron in the 2p‐orbital.

(c.) Zeff for an electron is ls‐orbital is different (d.) The two electrons present in the 2s‐orbital
from that of an electron in 2s‐orbital. have different quantum number values with
same sign.

(4.) Which of the following series of transitions in the spectrum of hydrogen atom falls in the visible region)
[NEET‐2019, Page: 45]
(a.) Lyman series (b.) Balmer series

(c.) Paschen series (d.) Brackett series

(5.) The probability density plots of ls and 2s orbitals are given in figure: [NCERT Exemplar modified,
Pages: 57‐59]

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 The density of dots in a region represents the probability density of finding electrons in the
region. On the basis of above diagram which of the following statements is/are correct)
(i)ls and 2s orbitals are spherical in shape.
(ii)The probability of finding the electron is minimum near the nucleus.
(iii)The probability of finding the electron at a given distance is equal in all directions.
(iv)The probability density of electrons for 2s‐orbital decreases uniformly as distance from the nucleus
increases.
(a.) (i) & (ii) only (b.) (ii) a (iii) only

(c.) (i) & (iii) only (d.) only (iv)

(6.) The energy of a mole of photons of radiation whose frequency is 5.5 1012 Hz is [Page: 41]
(a.) 2.18 J (b.) 2.18kJ

(c.) 2.18 eV (d.) 1.89kJ

(7.) The aim of Millikan’s oil drop experiment is to determine [Page: 32]
(a.) mass of electron (b.) velocity of electron
(c.) charge of electron (d.) elm of an electron

(8.) The energy associated with the first orbit of Li 2 is [Page: 48]
(a.) 19.62 1017 J (b.) 1.96 1017 J

(c.) 8.72 1018 J (d.) 2.18 1018 J

(9.) The number of electrons and neutrons of an element is 18 and 20 respectively. Its mass number is f Page:
35]
(a.) 18 (b.) 38

(c.) 20 (d.) 37

(10.) Few sets of quantum numbers are given below. Which of the following sets are incorrect? [NCERT
Exemplar Modified, HOT, Page: 57]
(I) n  1,l  1,m1  2
(II) n  2,l  1,m1  1
(III) n  3,l  2,m1  2
(IV) n  3,l  4,m1  2
(a.) (II) & (I1I) only (b.) (I) and (IV) only

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(c.) (I), (II) & (III) only (d.) All of these

(11.) A ball has a mass of 20 g and a speed of 45 m/s. It speed can be measured within the accuracy of 0.5%.
The uncertainty in its position will be [Page: 5l]
(a.) 1.49 1032 m (b.) 1.17 1032 m

(c.) 2.82 1033 m (d.) 5.12 1034 m

(12.) The energy of an electron in the 3rd orbit of hydrogen atom is ‐E. The energy of an electron in the first
orbit will be [Page: 47]
(a.) 9E (b.) 3E

(c.) E / 9 (d.) E / 3

(13.) In a hydrogen spectrum if electron moves from 5th to 2nd by transition in multi‐steps then find the number
of lines in spectrum. [Page: 45]
(a.) 10 (b.) 6

(c.) 2 (d.) 8

(14.) The number of photons which will provide 2 J of energy and having wavelength 7500 A is approx.
[Page: 4l]
(a.) 8 1018 (b.) 2 1019

(c.) 9 1017 (d.) 8 1019

(15.) Assertion: Greater the magnitude of the negative charge on the particle, greater is the interaction with
electric or magnetic field and thus greater the deflection.
Reason: The deflection of electrons from its original path decreases with the increase in the voltage across
the electrodes. [Page: 3l]
(a.) Both A and R are true and R is the correct (b.) Both A and R are true but R is not the correct
explanation of A. explanation of A.

(c.) A is true but R is false. (d.) Both A and R are false.

(16.) Match the following species with their corresponding ground state electronic configuration. [NCERT
Exemplar Modified, Page: 63]

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 Atom / Ion Electronic configuration
i  Cu P 1s 2 2s 2 2p6 3s 2 3p 6 3d10
(ii) Cu  Q 1s 2 2s 2 2p6 3s 2 3p 6
 iii  Fe3 R  1s 2 2s 2 2p6 3s 2 3p 2 3d10 4s1
 iv  Sc3  S 1s 2 2s 2 2p6 3s 2 3p6 3d 9 4s1
T 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5
(a.) (i)‐R, (ii) ‐ P, (iii)‐T, (iv)‐Q (b.) (i)‐R, (ii) ‐ P, (iii) ‐ S, (iv) ‐T

(c.) (i)‐P, (ii)‐ R, (iii)‐ S, (iv)‐T (d.) (i)‐P, (ii)‐Q, (iii)‐ T, (iv)‐S

(17.) Match the following: [Page: 35]

Column ‐ 1 Column ‐ 11
A. Isotope (i) 86 89
36 Kr,39 Y

B. Isobar (ii) 127 131

53 I , 53 I

C. Isotone (iii) 21Sc3 17Cl 

D. Isoelectronic (iv)1490 K,16
40
S
(a.) A‐(iii), B‐(iv),C‐(ii), D‐(i) (b.) A‐(ii), B‐(iv), C‐(i), D‐(iii)

(c.) A‐ (ii), B‐(i), C‐ (iv), D‐ (iii) (d.) A‐ (iii), B‐ (ii), C‐(i), D‐ (iv)

(18.) The specific charge for positive rays is X and that of cathodic rays is Y then [Page: 3l]
(a.) X  Y (b.) Y  X

(c.) XY (d.) X can be greater or less than Y.

(19.) Write the complete symbol of an element with number of protons  56 and mass number 138. [Page: 35]
138 138
(a.) 56 Ba (b.) 56 Fe

82 138
(c.) 56 Ba (d.) 82 Fe

(20.) Which of following is responsible to rule out the existence of definite paths or trajectories of electrons.
[NCERT Exemplar Modified, Page: 62]
(a.) Zeeman effect (b.) Heisenberg’s uncertainty principle

(c.) Hund’s rule of maximum multiplicity (d.) None of these

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