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metal because the gold has the highest malleability property. So the other
foil than gold would be thicker than gold foil which makes the more alpha
particles bounce back, making it uncertain where the positive mass of the
atom is located.
Q. Who discovered the neutrons?
Ans. In 1932, J. Chadwick discovered the neutrons.
Q. What are the properties of an electron?
Ans. An electron has the following properties:-
1. It is a negatively charged particle.
2. It is represented as ‘e
-
‘.
3. One electron has the charge which is equivalent to 1.6 X 10
-19
Coulomb.
4. One electron has the mass u.
1
2000
5. Electrons revolve around the nucleus of an atom.
Q. What are the properties of a proton?
Ans. A proton has the following properties:-
1. It is a positively charged particle.
2. It is represented as ‘p+‘.
3. One proton has the charge which is equivalent to 1.6 10
-19
×
Coulomb.
4. One proton has the mass 1 u.
5. Protons reside in the nucleus of an atom.
Q. What are the properties of a neutron?
Ans. A neutron has the following properties:-
1. It is an electrically neutral charged particle.
2. It is represented as ‘n‘.
3. A neutron has the charge which is equivalent to 0 C.
4. One neutron has the mass 1 u.
5. Neutrons reside in the nucleus of an atom.
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Q. How is the mass of an atom obtained?
Ans. The mass of an atom is obtained by the sum of the masses of protons
and neutrons present in the nucleus.
Q. Helium atom has an atomic mass of 4 u and two protons in its nucleus.
How many neutrons does it have?
Ans. Atomic mass = Number of protons + Number of neutrons
4 = 2 + Number of neutrons
Number of neutrons = 4 - 2
Number of neutrons = 2 Ans.
Q. How are Electrons Distributed in Different Orbits (Shells)?
Ans. The distribution of electrons into different orbits of an atom was
suggested by Bohr and Bury.
The following rules are followed for writing the number of electrons in
different energy levels or shells:
1. The maximum number of electrons present in a shell is given by the
formula 2n
2
, where ‘n’ is the orbit number.
2. The maximum number of electrons that can be accommodated in the
outermost orbit is 8.
3. Electrons are not accommodated in a given shell, unless the inner
shells are filled. That is, the shells are filled in a stepwise manner.
Q. Draw the atomic structure of the first eighteen elements.
Ans.
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Q. What does valence electrons mean?
Ans. The electrons present in the outermost shell of an atom are known as
the valence electrons. For example valence electrons of oxygen is 6,
because it has 6 electrons in its outermost shell.
Q. What does inert element mean?
Ans. The elements which the outermost shell completely filled, are known
as inert elements or inert gas or noble gas. They are very less or none
reactive. Examples of noble gasses are He, Ne, Ar etc.
Q. What does valency of an atom mean?
Ans. All atoms react with other atoms to completely fill its outermost shell.
This was done by sharing, gaining or losing electrons. The number of
electrons gained, lost or shared so as to fill its outermost shell, is known as
the valency of the atom. For example :- Oxygen will gain 2 e
-
to fill its
outermost shell and Aluminium will lose 3e
-
to fill its outermost shell, so
their valencies are respectively 2 and 3. The atoms which gain e
-
form
anion and which lose e
-
form cation such as O
2-
and Al
3+
.
Q. How will you find the valency of chlorine, sulphur and magnesium?
Ans. For finding the valency of any atom, we should write its electronic
configuration for example the electronic configuration of chlorine, sulphur
and magnesium are respectively :- (2,8,7) , (2,8,6) and (2,8,2). To fill the
outermost shell chlorine and sulphur need to gain respectively 1e
-
and 2e
-
so their valencies are 1 and 2 but for magnesium, it needs to gain 6e
-
or to
lose 2e
-
. Any atom fills its outermost shell by loosing, gaining or sharing
whichever is fast and easy. For magnesium it is easier to lose 2e
-
than to
gain 6e
-
so the valency of magnesium is 2.
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Q. What does the atomic number of an atom mean?
Ans. The atomic number of an atom is defined as the total number of
protons present in the nucleus of an atom. It is denoted by ‘Z’. For
example, Oxygen has the atomic number = 8 because it has 8 protons in its
nucleus.
Q. What does the mass number of an atom mean?
Ans. The mass number of an atom is defined as the sum of the total
number of protons and neutrons present in the nucleus of an atom. It
is denoted by ‘A’. For example, Oxygen has the mass number = 16(8+8)
because it has 8 protons and 8 neutrons in its nucleus.
Q. How to represent an atom with its symbol, atomic number and mass
number?
Ans. In the notation for an atom, the atomic number, mass number and
symbol of the element are to be written as:
For example, nitrogen is written as
14
7
N.
Q. If the number of electrons in an atom is 8 and the 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?
Ans. The atomic number of the atom is 8 because it has 8 protons. And it
has no charge because the positive charges and negative charges are
equal in magnitude which cancel out each other.
Q. What do isotopes mean?
Ans. Isotopes are defined as the atoms of the same element, having the
same atomic number but different mass numbers. For example carbon has
two isotopes because there are two types of carbon found in nature, one
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has mass number 12 and other has 14 which are represented as
respectively
6
12
C and
6
14
C.
Q. How do isotopes of an atom differ from each other and how are they
alike?
Ans. Isotopes of an atom differ from each other in the following manner:-
1. They have different mass numbers.
2. They have different numbers of protons.
3. They have different physical properties.
Isotopes of an atom are alike from each other in the following manner:-
1. They have the same atomic number.
2. They have the same number of protons.
3. They have the same number of electrons.
4. They have the same chemical properties.
Q. What do Protium, Deuterium and Tritium mean?
Ans. Hydrogen atom has three isotopes:-
1. Protium(H):-
1
1
H
2. Deuterium:-
1
2
H
3. Tritium:-
1
3
H
Q. Which atomic mass is taken in the case of isotopes?
Ans. Average atomic mass is taken in the case of isotopes, which is
calculated as:- m
1
amount% + m
2
amount% + m
3
amount% …….
× × ×
Q. Calculate the average atomic mass of chlorine which occurs in nature in
two isotopic forms, with masses 35 u and 37 u in the ratio of 3:1.
Ans. Chlorine has two isotopic forms, with masses 35 u and 37 u in the
ratio of 3:1. First ratio is to be converted into percentage:-
3
4
× 100 𝑎𝑛𝑑
1
4
× 100 = 75% 𝑎𝑛𝑑 25% .
Average atomic mass = m
1
amount% + m
2
amount%
× ×
=
35 × 75% + 37 × 25%
=
35 ×
75
100
+ 37 ×
25
100
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=
2625
100
+
925
100
= 26 . 25 + 9 . 25
= Ans.
35 . 5 𝑢
Q. What are the applications of isotopes of an atom?
Ans. Some isotopes have special properties which find them useful in
various fields. Some of them are :
1. An isotope of uranium is used as a fuel in nuclear reactors.
2. An isotope of cobalt is used in the treatment of cancer.
3. An isotope of iodine is used in the treatment of goitre.
Q. What does isobar mean?
Ans. Atoms of different elements with different atomic numbers, which
have the same mass number, and are known as isobars. For example
Argon and Calcium are isobars because they have the same mass number
which is 40.
Q. For the symbol H,D and T tabulate three sub-atomic particles found in
each of them.
Ans.
Electrons
Protons
Neutrons
H
1
1
0
D
1
1
1
T
1
1
2
Q. Write the electronic configuration of any one pair of isotopes and
isobars.
Ans. One pair of isotopes are :-
6
12
C and
6
14
C.
Electronic configuration of
6
12
C = (2,4)
Electronic configuration of
6
14
C = (2,4)
One pair of isobars are:-
18
40
Ar and
20
40
Ca.
Electronic configuration of
18
40
Ar = (2,8,8)
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Electronic configuration of
20
40
Ca = (2,8,8,2)
Q. What are the drawbacks of Dalton's Atomic Theory?
Ans. Some drawbacks of Dalton's Atomic Theory are:-
1. Subatomic Particles:- According to Dalton's Atomic Theory, an atom
is the smallest particle but the discovery of subatomic particles
proved it wrong.
2. Isotopes:- According to Dalton's Atomic Theory, atoms of the same
element have the same mass but the discovery of isotopes, which
says that atoms of the same element may have the different masses,
proved it wrong.
3. Isobars:- According to Dalton's Atomic Theory, atoms of different
elements have different masses but the discovery of isobars, which
says that atoms of different elements may have the same masses,
proved it wrong.
Exercises
Q. Compare the properties of electrons, protons and neutrons.
Ans.
Electrons
Protons
Neutrons
It is a negative particle.
It is a positive particle.
It is a neutral particle.
It has 1.6 10
-19
C
×
charge.
It has 1.6 10
-19
C
×
charge.
It has a 0C charge.
It is denoted as ‘e
-
’.
It is denoted as ‘p
+
’.
It is denoted as ‘n’.
Its mass = .
1
2000
𝑢
Its mass = .
1 𝑢
Its mass = .
1 𝑢
It revolves around the
nucleus.
It resides in the
nucleus.
It resides in the
nucleus.
It was discovered by J.
J. Thomson.
It was discovered by E.
Goldstein
It was discovered by
James Chadwick.
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Q. What are the limitations of Rutherford’s model of the atom?
Ans. He could not explain the stability of an atom. According to his model
of an atom, the revolution of the electron in a circular orbit is not expected
to be stable. Any particle in a circular orbit would undergo acceleration.
During acceleration, charged particles would radiate energy. Thus, the
revolving electron would lose energy and finally fall into the nucleus. If this
were so, the atom should be highly unstable and hence matter would not
exist in the form that we know. We know that atoms are quite stable.
Q. Describe Bohr’s model of the atom.
Ans. Neils Bohr put forward the following postulates about the model of an
atom:
1. Only certain special orbits known as discrete orbits of electrons, are
allowed inside the atom.
2. While revolving in discrete orbits the electrons do not radiate energy.
Q. Compare all the proposed models of an atom given in this chapter.
Ans.
Thomson’s model
Rutherford’s model
Bohr’s model
Thomson proposed
that an atom contains
negatively charged
particles embedded in
a positively charged
sphere.
Rutherford proposed
that an atom has a
positively charged
nucleus at its center,
where most of the
atom's mass is located.
Electrons revolve
around the nucleus in
circular paths.
Bohr proposed that an
atom has a positively
charged nucleus at its
center, where most of
the atom's mass is
located and electrons
revolve around the
nucleus in orbits with
fixed energy shells.
Q. Summarise the rules for writing of distribution of electrons in various
shells for the first eighteen elements.
Ans. The following rules are followed for writing the number of electrons in
different energy levels or shells:
1. The maximum number of electrons present in a shell is given by the
formula 2n
2
, where ‘n’ is the orbit number.
SMART EDUCATIONS( https://smarteducations.org/ ) 13
2. The maximum number of electrons that can be accommodated in the
outermost orbit is 8.
3. Electrons are not accommodated in a given shell, unless the inner
shells are filled. That is, the shells are filled in a stepwise manner.
Q. Define valency by taking examples of silicon and oxygen.
Ans. All atoms react with other atoms to completely fill its outermost shell.
This was done by sharing, gaining or losing electrons. The number of
electrons gained, lost or shared so as to fill its outermost shell, is known as
the valency of the atom. For example :- Si needs to share 4e
-
to fill its
outermost shell so its valency is 4 and Oxygen needs to gain 2e
-
to fill its
outermost shell so its valency is 2.
Q. Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes
and iv) Isobars. Give any two uses of isotopes.
Ans. Atomic number :- The atomic number of an atom is defined as the total
number of protons present in the nucleus of an atom. It is denoted by ‘Z’.
For example, Oxygen has the atomic number = 8 because it has 8 protons
in its nucleus.
Mass number :- The mass number of an atom is defined as the sum of
the total number of protons and neutrons present in the nucleus of an atom.
It is denoted by ‘A’. For example, Oxygen has the mass number = 16(8+8)
because it has 8 protons and 8 neutrons in its nucleus.
Isotopes :- Isotopes are defined as the atoms of the same element,
having the same atomic number but different mass numbers. For example
carbon has two isotopes because there are two types of carbon found in
nature, one has mass number 12 and other has 14 which are represented
as respectively
6
12
C and
6
14
C.
Q. Na+ has completely filled K and L shells. Explain.
Ans. The electronic configuration of Na = (2,8,1) but after losing 1 e
-
to fill
its outermost shell it forms Na
+
which has the electronic configuration =
(2,8). Hence Na+ has completely filled K and L shells.
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Q. If a bromine atom is available in the form of, say, two isotopes
79
35
Br
(49.7%) and
35
81
Br (50.3%), calculate the average atomic mass of the
bromine atom.
Ans. Average atomic mass = m
1
amount% + m
2
amount%
× ×
=
79 × 49 . 7% + 81 × 50 . 3%
=
79 ×
49 . 7
100
+ 81 ×
50 . 3
100
=
3926 . 3
100
+
4074 . 3
100
=
39 . 263 + 40 . 743
= Ans. 80 . 006 𝑢
Q. The average atomic mass of a sample of an element X is 16.2 u.
What are the percentages of isotopes
8
16
X and
8
18
X in the sample?
Ans. Let
8
16
X = y% and
8
18
X = (100-y)%
Average atomic mass = m
1
amount% + m
2
amount%
× ×
16.2 =
16 × 𝑦 % + 18 × 100 − 𝑦 ( ) %
16.2 =
16 ×
𝑦
100
+ 18 ×
100 − 𝑦 ( )
100
16.2 =
16 𝑦
100
+
1800 − 18 𝑦
100
16.2 =
16 𝑦 + 1800 − 18 𝑦
100
16 . 2 × 100 = − 2 𝑦 + 1800
1620 − 1800 = − 2 𝑦
𝑦 =
− 180
− 2
𝑦 = 90
Hence
8
16
X = 90% and
8
18
X = 10% Ans.
Q. If Z = 3, what would be the valency of the element? Also, name the
element.
Ans. If Z = 3, then the atom has 3 protons and hence 3 electrons. So
electronic configuration of the atom = (2,1) so it needs to lose 1 e
-
to fill its
outermost shell so its valency is 1. Name of the atom is Lithium.
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Q. Composition of the nuclei of two atomic species X and Y are given as
under
X Y
Protons = 6 6
Neutrons = 6 8
Give the mass numbers of X and Y. What is the relation between the two
species?
Ans. Mass number of X = 6 + 6
= 12
Mass number of Y = 6 + 8
= 14
Since X and Y have the same number of protons(6) so they have the same
atomic number but they have the different mass number 12 and 14
respectively. So, X and Y are isotopes.
Q. For the following statements, write T for True and F for False.
1. J.J. Thomson proposed that the nucleus of an atom contains only
nucleons.
Ans. F
2. A neutron is formed by an electron and a proton combining together.
Therefore, it is neutral.
Ans. F
3. The mass of an electron is about times that of protons.
1
2000
Ans. T
4. An isotope of iodine is used for making tincture iodine, which is used
as a medicine.
Ans. F
Q. Rutherford’s alpha-particle scattering experiment was responsible for
the discovery of:-
Ans. Atomic Nucleus
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Q. Isotopes of an element have:-
Ans. different number of neutrons
Q. Number of valence electrons in Cl
–
ion are:-
Ans. 8
Q. Which one of the following is a correct electronic configuration of
sodium?
Ans. 2,8,1
Q. Complete the following table.
Ans.
