CHAPTER 4: STRUCTURE OF THE ATOM

NOTES

Atoms
● Atoms are the basic building blocks of matter.
● Different kinds of matter exist because there are different kinds of atoms present in them.
Charged Particles in Matter
● Whenever we rub two objects together, they become electrically charged. This is because
atoms contain charged particles in them. Therefore, atoms can be divided further into particles
i.e. proton, electron and neutron.
● Atoms consist of protons and electrons in a balanced proportion.
● Protons exist in the interiors of the atom and electrons exist in the exteriors of the atom.
Therefore, electrons can be removed from an atom.

Failure of Dalton’s Atomic Theory

Dalton suggested that atoms can neither be created nor destroyed and are indivisible. But the discovery
of electrons and protons in atoms lead to failure of this aspect of Dalton’s theory.

Thomson’s Model of an Atom

According to J.J. Thomson, the structure of an atom can be compared to

● Christmas pudding: The electrons, in a sphere of positive charge, were like currants (dry fruits)
in a spherical Christmas pudding.
● Watermelon: the positive charge in the atom is spread all over like the red edible part while the
electrons are studded in the positively charged sphere, like the seeds.
Thomson proposed that:
(i) An atom consists of a positively charged sphere and the electrons are embedded in it.
(ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically
neutral

Rutherford’s Model of an Atom

Rutherford’s Experiment

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 ● He experimented with thin gold foil by passing alpha rays through it.
● He expected that the gold atoms will deflect the Alpha particles.

Observations Inferences

Alpha particles which had high speed Atom contains a lot of empty space
moved straight through the gold foil

Some particles got diverted a by slide Positive charges in the atom are not occupying much of
angles its space

Only one out of 12000 particles bounced A very small fraction of α-particles were deflected by
back 180⁰, indicating that all the positive charge and mass of
the gold atom were concentrated in a very small
volume within the atom.
Thus, Rutherford gave the nuclear model of an atom based on his experiment which suggests that -
● There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an
atom resides in the nucleus.
● The electrons revolve around the nucleus in circular paths.
● The size of the nucleus is very small as compared to the size of the atom.

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Drawbacks of the Nuclear Model of an Atom

● He explained that the electrons in an atom revolve around the nucleus in well-defined orbits.
Particles in a circular orbit would experience acceleration.
● Thus, the revolving electron would lose energy and finally fall into the nucleus.
● But this cannot take place as the atom would be unstable, and the matter would not exist in the
form we know.
Nucleons – Protons and Neutrons are collectively called as Nucleons.

Bohr's Model of an Atom

Bohr suggested that –

● Electrons revolve around the nucleus in stable orbits without the emission of radiant energy. Each
orbit has a definite energy and is called an energy shell or energy level.
● An orbit or energy level is designated as K, L, M, and N shells. When the electron is in the lowest
energy level, it is said to be in the ground state.
● An electron emits or absorbs energy when it jumps from one orbit or energy level to another.
● They are represented using letters or numbers as shown in the figure below –

The Neutrons

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J. Chadwick discovered that there is another sub-atomic particle present in the atom. This particle carries
no charge and is known as a Neutron. Therefore, we can conclude that atom consists of three types of
particles –

Electrons which carry a negative charge

Protons which carry a positive charge
Neutrons they are neutral
The distribution of electrons in different shells or orbits
● If Orbit number = n
● Then number of electrons present in an Orbit = 2n2
● So for (n =1) first orbit or K-shell will be = 2 × 12 = 2,

● for (n =2) second orbit or L-shell will be = 2 × 22 = 8,

● for (n=3) third orbit or M-shell will be = 2 × 32 = 18,
● for (n =4) fourth orbit or N-shell will be = 2 × 42 = 32
● The outermost shell can contain at most 8 electrons.

● The shells in an atom are filled in sequence.

● Thus, until the inner shells of an atom are filled completely the outer shells cannot contain any
electrons.
Valency
● Valence Electrons – The electrons present in the outermost shell of an atom are known as the
valence electrons.
● The atoms which have completely filled the outermost shell are not very active chemically.

● The valency of an atom or the combining capacity of an atom is given by the number of elements
present in the outermost shell.
● For Example, hydrogen/ lithium/sodium atoms contain one electron each in their outermost
shell, therefore each one of them can lose one electron. So, they are said to have valency of one.
● What happens when the outermost shell contains a number of electrons that are close
to its maximum capacity?
Valency in such cases is generated by subtracting the number of electrons present in the outermost orbit
from octet (8). For example, oxygen contains 6 electrons in its outermost shell. Its valency is calculated
as: 8 – 6 = 2. This means oxygen needs two electrons to form a bond with another element.
Atomic Number of an Element
Atomic Number (Z) = Number of protons in an atom
Mass Number of an Element
Mass Number = Number of protons + Number of neutrons

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Isotopes

● The atoms of an element can exist in several forms having similar atomic numbers but varying
mass numbers.
● Isotopes are pure substances.
● Isotopes have a similar chemical nature.
● Isotopes have distinct physical characteristics.

Where can we use Isotopes?

1. The fuel of Nuclear Reactor – Isotope of Uranium
2. Treatment of Cancer – Isotope of Cobalt
3. Treatment of Goiter – Isotope of Iodine

Chlorine occurs in nature in two isotopic forms, with masses 35 u and 37 u in the ratio of 3:1.
What should we take as the mass of chlorine atom?

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Isobars

The atoms of several elements can have a similar mass number but distinct atomic masses. Such elements
are called Isobars.

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 QUESTION BANK

General Instructions:
(i) Section A contains multiple-choice and assertion reason type questions carrying one mark each.
(ii) Section B contains very short answer type questions carrying two marks each. Answers to these
questions should be in the range of 30 to 50 words.
(iii) Section C contains short answer type questions carrying three marks each. Answers to these
questions should be in the range of 50 to 80 words.
(iv) Section D contains case-based questions.

SECTION–A
1. An atom with 3 protons and 4 neutrons will have a valency of:
a. 3
b. 7
c. 1
d. 4
Answer:
c. 1
2. The electron distribution in an aluminium atom is:
a. 2,8,3
b. 2,8,2
c. 8,2,3
d. 2,3,8
Answer:
a. 2,8,3
3. Which of the following in Fig. do not represent Bohr’s model of an atom correctly?

a. (i) and (ii)

b. (ii) and (iii)
c. (ii) and (iv)
d. (i) and (iv)
Answer:
c. (ii) and (iv)
4. Which of the following statement is always correct?
a. An atom has equal number of electrons and protons.
b. An atom has equal number of electrons and neutrons.
c. An atom has equal number of protons and neutrons.
d. An atom has equal number of electrons, protons and neutrons.
Answer:
a. An atom has equal number of electrons and protons.

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 5. Atomic models have been improved over the years. Arrange the following atomic models in the
order of their chronological order
(i) Rutherford’s atomic model
(ii) Thomson’s atomic model
(iii) Bohr’s atomic model
a. (i), (ii) and (iii)
b. (ii), (iii) and (i)
c. (ii), (i) and (iii)
d. (iii), (ii) and (i)
Answer:
c. (ii), (i) and (iii)
6. Assertion: Isotopes are electrically neutral.
Reason: Isotopes are species with same mass number but different atomic numbers
a. Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion
(A).
b. Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of
assertion (A).
c. Assertion (A) is true but reason (R) is false.
d. Assertion (A) is false but reason (R) is true.
Answer:
c. Assertion (A) is true but reason (R) is false.
7. Assertion: Atom is electrically neutral.
Reason: A neutral particle, neutron is present in the nucleus of atom.
a. Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion
(A).
b. Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of
assertion (A).
c. Assertion (A) is true but reason (R) is false.
d. Assertion (A) is false but reason (R) is true.
Answer:
b. Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of
assertion (A).
8. Assertion: The size of the nucleus is very small as compared to the size of the atom.
Reason: The electrons revolve around the nucleus of the atom
a. Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion
(A).
b. Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of
assertion (A).
c. Assertion (A) is true but reason (R) is false.
d. Assertion (A) is false but reason (R) is true.
Answer:
b. Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of
assertion (A).
9. Why did Rutherford select a gold foil in his α–ray scattering experiment?
Answer:
Gold is a heavy metal with high mass number. A light metal cannot be used because on being hit
by fast moving α-particles, the atom of light metal will be simply pushed forward and no
scattering can occur. Moreover, gold is highly malleable and can be beaten to get very thin foils.

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 10. The atomic number of calcium and argon are 20 and 18 respectively, but the mass number of both
these elements is 40. What is the name given to such a pair of elements?
Answer:
A pair of elements in which the elements have same mass number but different atomic numbers
are called isobars. Hence 4020Ca and is 4018Ar are isobars.
11. Helium atom has 2 electrons in its valence shell but its valency is not 2, Explain.
Answer:
Helium has only one shell (K – shell) and the maximum number of electrons within K shell can be
2. Hence the valency is zero.
12. What were the drawbacks of Rutherford’s model of an atom?
Answer:
Drawbacks of Rutherford’s model of an atom:
(i) Any particle in a circular motion would undergo acceleration and would radiate energy. Thus,
revolving electron would lose energy and finally fall into the nucleus. Thus, atom should be
unstable and should not exist in the stable form.
(ii)The model did not give any arrangement of electrons around the nucleus.
13. In response to a question, a student stated that in an atom, the number of protons is greater than
the number of neutrons, which in turn is greater than the number of electrons. Do you agree with
the statement? Justify your answer.
Answer:
The given statement is not correct. As number of protons is never greater than number of
neutron. Number of neutrons can be equal to or greater than number of protons but number of
protons is equal to number of electrons for an atom since it is neutral.
14. The maximum number of the electrons which are permitted to be assigned to an energy shell of
an atom is called the electron capacity of that shell. The distribution of electrons in different
orbits or shell is governed by a scheme known as Bohr-Bury scheme. According to this scheme:
(a) The maximum number of the electrons that can be present in any shell is given by the formula
2n2 where, n is the number of energy level.
(b) The maximum number of electrons that can be accommodated in the outermost shell is 8.
Electrons are filled in the shells in a stepwise manner in increasing order of energy of the energy
shell.
a. What is the maximum electrons capacity of N shell?
b. Identify the element with the configuration K-2, L-8, M-3.
c. What are valence electrons?
d. Write the electronic configuration of the element with atomic number 17. Indicate the valency
of the atom.
Answer:
a. For N-shell, n = 4
Maximum electron capacity = 2n2 = 2 x 42 = 32
b. The element will be Aluminum.
c. The electrons present in the outermost shell of an atom are known as the valence electrons.
d. Atomic number = 17
Electronic configuration = K-2, L-8, M-7

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 NCERT SOLUTIONS

1. What are the canal rays?

Answer:
Canal rays are positively charged radiations which led to the discovery of positively charged
sub-atomic particle called proton.

2. If an atom contains one electron and one proton, will it carry any charge or not?

Answer:
The atom will be electrically neutral as one negative charge balances one positive charge
charge.

3. On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.

Answer:
According to Thomson’s model of an atom
(i) An atom consists of a positively charged sphere and the electrons are embedded in it,
(ii) The negative and positive charges are equal in magnitude. So the atom is electrically
neutral.

4. On the basis of Rutherford’s model of an atom, which subatomic particle is present in the
nucleus of an atom?

Answer:
As per Rutherford’s model of an atom, the protons which are positively charged are present
in the nucleus of an atom.

5. Draw a sketch of Bohr’s model of an atom with three shells.

Answer:

6. What do you think would be the observation if the α-particle scattering experiment is
carried out using a foil of a metal other than gold?

Answer:
The observation would remain the same. This is because the structure of an atom, when
considered individually, remains the same.

7. Name the three sub-atomic particles of an atom.

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 Answer:
An atom consists of three subatomic particles:

● Protons – Positively charged

● Electrons – Negatively charged
● Neutrons – Neutron (no charge)

8. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons
does it have?

Answer:
Atomic mass of helium atom = 4u
No. of protons= 2
Atomic number = No. of protons + No. of neutrons
4 = 2 + No. of neutrons
No. of neutrons = 4 – 2 = 2
Hence, Helium has 2 neutrons.

9. Write the distribution of electrons in carbon and sodium atoms.

Answer:

10. If K and L shells of an atom are full, then what would be the total number of electrons in the
atom?
Answer:

K shell can hold 2 electrons and L shell can hold 8 electrons. When both the shells are full,
there will be 8 + 2 = 10 electrons in the atom.

11. How will you find the valency of chlorine, sulphur and magnesium?

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 Answer:
Calculation of valency:

Valency of chlorine:

The electronic configuration of chlorine = 2, 8, 7

Chlorine has 7 (more than 4) electrons in its outermost shell.

Therefore, the valency of chlorine = 8 – the number of electrons in the outermost shell

= 8−7

=1

Valency of Sulphur:

The electronic configuration of Sulphur = 2, 8,6

Sulphur has 6 (more than 4) electrons in its outermost shell.

Therefore, the valency of chlorine = 8 – the number of electrons in the outermost shell

= 8−6

=2

Valency of magnesium:

The electronic configuration of Magnesium = 2, 8, 2

Magnesium has 2 (less than 4) electrons in its outermost shell.

Therefore, the valency of magnesium= Number of electrons in its outermost shell

=2

12. If number of electrons in an atom is 8 and number of protons is also 8, then (i) what is
the atomic number of the atom? and (ii) what is the charge on the atom?
Answer:

No. of electrons = 8
No. of protons = 8
(i) Atomic number = No. of protons = 8
(ii) No. of electrons = No. of protons
Hence, both the charges positive and negative neutralise each other. Therefore, the atom
does not possess any charge.

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 13. With the help of Table 4.1, find out the mass number of oxygen and sulphur atom.
Answer:

(a) To find the mass number of Oxygen,

Number of protons = 8

Number of neutrons = 8

Atomic number = 8

Atomic mass number = Number of protons + number of neutrons = 8 + 8 = 16

Therefore, the mass number of oxygen = 16

(b) To find the mass number of Sulphur,

Number of protons = 16

Number of neutrons = 16

Atomic number = 16

Atomic mass number = Number of protons + number of neutrons = 16 + 16 = 32

14. For the symbol H, D and T tabulate three sub-atomic particles found in each of them.
Answer:

The following table depicts the subatomic particles in Hydrogen (H), Deuterium (D), and
Tritium(T).

15. Write the electronic configuration of any one pair of isotopes and isobars.
Answer:

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 Isotopes: Atoms of same element having same atomic number but different mass number.

16. Compare the properties of electrons, protons and neutrons.

Answer:

17. What are the limitations of J.J. Thomson’s model of the atom?
Answer:

The following are the limitations of J.J. Thomson’s model of an atom:

● The model failed to explain the outcome of alpha particle scattering, which was
conducted by Rutherford. The model failed to depict why the majority of these alpha
particles pass through the gold foil, while some diverted through small and big
angles, while some others rebound completely, returning on their path.
● It did not provide any experimental evidence and was established on imagination.

18. What are the limitations of Rutherford’s model of the atom?

Answer:

The following are the limitations of Rutherford’s model of the atom:

● There is no expected stability in the revolution of the electron in a circular orbit.
● Charged particles radiate energy when accelerated, thus causing the revolving
electrons to lose energy and would fall into the nucleus.
● Hence, atoms must be highly unstable. The matter would not exist in its known form,
which clearly is an assumption as atoms are highly stable.
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 19. Describe Bohr’s model of the atom.
Answer:

Bohr’s model of the atom

(1) Atom has nucleus in the centre.
(2) Electrons revolve around the nucleus.
(3) Certain special orbits known as discrete orbits of electrons are allowed inside the atom.
(4) While revolving in discrete orbits the electrons do not radiate energy.
(5) These orbits or shells are called energy levels.
(6) These orbits or shells are represented by the letters K, L, M, N or the numbers n = 1, 2, 3,
4

20. Compare all the proposed models of an atom given in this chapter.
Answer:

21. Summarise the rules for writing of distribution of electrons in various shells for the
first eighteen elements.

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 Answer:

● The maximum number of electrons that can be accommodated in a shell is given by

the formula: 2n2, where n= 1, 2, 3…
● The maximum number of electrons in different shells are:
K shell – n=1 ; 2n2 = 2(1)2 = 2

L shell – n=2 ; 2n2 = 2(2)2 = 8

M shell – n=3 ; 2n2 = 2(3)2 = 18

N shell- n=4 ; 2n2 = 2(4)2 = 32

● The outermost orbit can be accommodated with 8 electrons at the maximum.

● The electrons are not taken in unless the inner shells are filled, which are filled step-
wise; hence, the highest element has K-2; L-8; M-8 distribution of electrons.

22. Define valency by taking examples of silicon and oxygen.

Answer:

Valency is the combining capacity of an atom.

Atomic number of oxygen = 8 Atomic number of silicon = 14 K L M
Electronic configuration of oxygen = 2 6
Electronic configuration of silicon =2 8 4
In the atoms of oxygen the valence electrons are 6 (i.e., electrons in the outermost shell). To
fill the orbit, 2 electrons are required. In the atom of silicon, the valence electrons are 4. To
fill this orbit 4 electrons are required.
Hence, the combining capacity of oxygen is 2 and of silicon is 4.
i.e., Valency of oxygen = 2
Valency of silicon = 4

23. Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and iv)
Isobars. Give any two uses of isotopes.
Answer:

(i) Atomic number: The atomic number of an element is equal to the number of protons in
the nucleus of its atom. e.g., Oxygen has 6 protons hence atomic no. = 6.
(ii) Mass number: The mass number of an atom is equal to the number of protons and
neutrons in its nucleus. It is denoted by A. Example: Protons + Neutrons= Mass number 6 +
6 = 12
(iii) Isotopes: Isotopes are atoms of the same element which have different mass number
but same atomic number.

(iv) Isobars: Isobars are atoms having the same mass number but different atomic numbers.

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 Both calcium and argon have same mass number but different atomic number.
Two uses of isotopes are:
(i) An isotope of iodine is used in the treatment of goitre.
(ii) An isotope of uranium is used as a fuel in nuclear reactors.

24. Na+ has completely filled K and L shells. Explain.

Answer:

Sodium atom (Na), has atomic number =11

Number of protons =11
Number of electrons = 11
Electronic configuration of Na = K L M – 2 8 1
Sodium atom (Na) looses 1 electron to become stable and form Na+ ion. Hence it has
completely filled K and L shells.

25. If bromine atom is available in the form of, say, two isotopes 79 35 Br (49.7%) and 81
35 Br (50.3%), calculate the average atomic mass of bromine atom.
Answer:

26. The average atomic mass of a sample of an element X is 16.2 u. What are the
percentages of isotopes 16 8 X and 18 8 X in the sample?
Answer:

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 Let the percentage of 168X be x and the percentage of 168X be 100 – x.

27. If Z = 3, what would be the valency of the element? Also, name the element.
Answer:

Z = 3, (i.e, atomic number —> z)

∴ Electronic configuration = 2, 1
Valency = 1
Name of the element is lithium.

28. Composition of the nuclei of two atomic species X and Y are given as under

Give the mass numbers of X and Y. What is the relation between the two species?
Answer:

Mass number of X = Protons + Neutrons

= 6 + 6 = 12
Mass number of Y = Protons + Neutrons = 6 + 8 = 14
As the atomic number is same i.e., = 6.
[atomic number = number of protons].
Both X and Y are isotopes of same element.

29. For the following statements, write T for True and F for False.
(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
(b) A neutron is formed by an electron and a proton combining together. Therefore, it
is neutral.

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 (c) The mass of an electron is about 12000 times that of proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a
medicine.
Answer:

(a) False (b) False

(c) True (d) False

Put tick (¸) against correct choice and cross (×) against wrong choice in questions 15,
16 and 17

30. Rutherford’s alpha-particle scattering experiment was responsible for the discovery
of
(a) Atomic Nucleus
(b) Electron
(c) Proton
(d) Neutron
Answer:

(a) Atomic nucleus

31. Isotopes of an element have

(a) the same physical properties
(b) different chemical properties
(c) different number of neutrons
(d) different atomic numbers.
Answer:

(c) different number of neutrons

32. Number of valence electrons in Cl– ion is:

(a) 16
(b) 8
(c) 17
(d) 18
Answer:

(b) 8

33. Which one of the following is a correct electronic configuration of sodium?

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 (a) 2,8
(b) 8,2,1
(c) 2,1,8
(d) 2,8,1
Answer:

(d) 2,8,1

34. Complete the following table.

Answer:

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General Instructions:
(i) Section A contains multiple-choice and assertion reason type questions carrying one mark each.
(ii) Section B contains very short answer type questions carrying two marks each. Answers to these
questions should be in the range of 30 to 50 words.
(iii) Section C contains short answer type questions carrying three marks each. Answers to these
questions should be in the range of 50 to 80 words.
(iv) Section D contains case-based questions.

WORKSHEET-4.1
SECTION- A

1. Which of the following correctly represent the electronic distribution in the Mg atom?

a. 3, 8, 1

b. 2, 8, 2

c. 1, 8, 3

d. 2, 8

2. Rutherford’s ‘alpha (α) particles scattering experiment’ resulted in to discovery of:

a. Electron

b. Proton

c. Nucleus in the atom

d. neutron

3. The number of electrons in an element X is 15 and the number of neutrons is 16. Which
of the following is the correct representation of the element?

a. 3115X

b. 3116X

c. 1615X

d. 1516X

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 4. Which of the following statements about Rutherford’s model of atom are correct?

(i) considered the nucleus as positively charged

(ii) established that the α–particles are four times as heavy as a hydrogen atom

(iii) can be compared to solar system

(iv) was in agreement with Thomson’s model

a. (i) and (iii)

b. (ii) and (iii)

c. (i) and (iv)

d. only (i)

5. Which of the following are true for an element?

(i) Atomic number = number of protons + number of electrons

(ii) Mass number = number of protons + number of neutrons

(iii) Atomic mass = number of protons = number of neutrons

(iv) Atomic number = number of protons = number of electrons

a. (i) and (ii)

b. (i) and (iii)

c. (ii) and (iii)

d. (ii) and (iv)

6. In the Thomson’s model of atom, which of the following statements are correct?

(i) the mass of the atom is assumed to be uniformly distributed over the atom

(ii) the positive charge is assumed to be uniformly distributed over the atom

(iii) the electrons are uniformly distributed in the positively charged sphere

(iv) the electrons attract each other to stabilise the atom

a. (i), (ii) and (iii)

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 b. (i) and (iii)

c. (i) and (iv)

d. (i), (iii) and (iv)

7. Rutherford’s α–particle scattering experiment showed that (i) electrons have negative
charge (ii) the mass and positive charge of the atom is concentrated in the nucleus (iii)
neutron exists in the nucleus (iv) most of the space in atom is empty Which of the above
statements are correct?

a. (i) and (iii)

b. (ii) and (iv)

c. (i) and (iv)

d. (iii) and (iv)

8. The ion of an element has 3 positive charges. Mass number of the atom is 27 and the
number of neutrons is 14. What is the number of electrons in the ion?

a. 13

b. 10

c. 14

d. 16
9. In a sample of ethyl ethanoate (CH3COOC2H5) the two oxygen atoms have the same
number of electrons but different number of neutrons. Which of the following is the
correct reason for it?

a. One of the oxygen atoms has gained electrons

b. One of the oxygen atoms has gained two neutrons

c. The two oxygen atoms are isotopes

d. The two oxygen atoms are isobars.

10. Elements with valency 1 are:

a. always metals

b. always metalloids

c. either metals or non-metals

d. always non-metals

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 11. Assertion: For noble gases, valency is zero.
Reason: Noble gases have 8 valence electrons.
a. Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of
assertion (A).
b. Both assertion (A) and reason (R) are true but reason (R) is not the correct
explanation of assertion (A).
c. Assertion (A) is true but reason (R) is false.

d. Assertion (A) is false but reason (R) is true.

12. Assertion: Thomson’s atomic model is known as ‘raisin pudding’ model.

Reason: The atom is visualized as a pudding of positive charge with electrons (raisins)
embedded in it.
a. Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of
assertion (A).
b. Both assertion (A) and reason (R) are true but reason (R) is not the correct
explanation of assertion (A).
c. Assertion (A) is true but reason (R) is false.

d. Assertion (A) is false but reason (R) is true.

13. Assertion: The mass of the total number of protons and neutrons is a measure of the
approximate mass of an atom.
Reason: The mass of an electron is negligible.
a. Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of
assertion (A).
b. Both assertion (A) and reason (R) are true but reason (R) is not the correct
explanation of assertion (A).
c. Assertion (A) is true but reason (R) is false.

d. Assertion (A) is false but reason (R) is true.

SECTION– B
14. Is it possible for the atom of an element to have one electron, one proton and no neutron.
If so, name the element.
15. Write any two observations which support the fact that atoms are divisible.
16. Will 35Cl and 37Cl have different valencies? Justify your answer.

SECTION– C

17. Given that the percentage abundance of the isotope 2010Ne is 90% and that of the

isotope of 2210Ne is 10%. Calculate the average atomic mass of Neon.

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 18. Show diagrammatically the electron distributions in a sodium atom and a sodium ion
and also give their atomic number.

SECTION– D
19. James Chadwick, J.J. Thomson and E. Goldstein had discovered three sub atomic particles
P, Q and R, respectively. They gave the characteristics of all these particles, their
locations and their arrangements in the atom. Ernest Rutherford and Neils Bohr had also
contributed in this regard. Now, Answer the following questions:
a. What is the name of Particle P? What is the nature of charge on it. State its location in
the atom.
b. What is the name of Particle Q? What is the nature of charge on it. State its location in
the atom.
c. What is the name of Particle R? What is the nature of charge on it. State its location in
the atom.
d. Differentiate between P, Q and R on the basis of their mass.

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