# TEXTBOOK ANSWERS AND SOLUTIONS OF CBSE CLASS IX SCIENCE Chapter 4 Structure Of The Atom

1) Compare the properties of
electrons, protons and neutrons.
Negatively charged particles
Protons-Protons are
Positively charged particles.
Neutron-Neutrons do not
carry any charge and are neutral
2) What are the limitations of
J.J. Thomson’s model of the
atom?
model of the atom
are: It could not explain the result of scattering experiment performed
by Rutherford.
It did not have any experiment support.
3) What are the limitations of Rutherford’s model of the atom?
ANSWER;-The limitations of Rutherford’s model of the atom are It failed to explain the stability of an atom.
It doesn’t explain the spectrum of hydrogen and other
atoms.
4) Describe Bohr’s model of the atom.
ANSWER:- The atom consists of a small positively charged nucleus at its
center.
The whole mass of the
atom is concentrated at the nucleus and the volume of the nucleus is much
smaller than the volume of the atom.
All the protons and
neutrons
of the atom are contained
in
the nucleus.
Only certain orbits
known as
discrete orbits of
electrons are
allowed inside the atom.
while revolving in these discrete orbits electrons do not radiate energy. These orbits or cells
are represented by the
letters K, L, M, N etc.
6) Summarize the rules for writing of distribution of electrons in various shells for the
first eighteen
elements.
ANSWER:- If n gives the number of orbit or energy level, then 2n 2 gives the maximum number of electrons
possible in a given
orbit or energy level.
Thus,
First orbit or K-shell will have 2 electrons, Second orbit or L-shell will have 8 electrons, Third orbit or M-shell will have 18 electrons.
If it is the outermost
orbit, then it should have not more
than
8 electrons.
There should be
step-wise
filling of electrons in
different
orbits, i.e., electrons
are not
accompanied in a given
orbit if
the earlier orbits or
shells are
incompletely filled.
7) Define valency by taking examples of silicon and oxygen.
is determined by the
number of valence
electrons
present in the atom of
that
element. Valency of Silicon: It has electronic configuration: 2, 8, 4 Thus, the valency of silicon is 4 as these electrons can
be
shared with others to complete octet.
Valency of Oxygen: It
has
electronic configuration: 2, 6
Thus, the valency of oxygen is 2
as it will gain 2 electrons to
complete its octet.
8) 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
the total number of protons
present in the atom of
that
element. For example, nitrogen has 7 protons in its atom. Thus,
the atomic number of
nitrogen is 7.
(ii) Mass number: The mass number
of an element is the
sum of the number of
protons
and neutrons present in
the
atom of that element. For example, the atom of boron has
5 protons and 6 neutrons.
So, the mass number of boron is
5 + 6 = 11.
(iii) Isotopes: These are atoms of
the same element having
the same atomic number,
but
different mass numbers.
(iv) Isobars: These are atoms having
the same mass number,
but different atomic
numbers
i.e., isobars are atoms
of
different elements having the same mass number.
Two uses of isotopes:
One isotope of uranium
is
used as a fuel in nuclear
reactors.
One isotope of cobalt is used in the treatment of cancer.
9) Na + has completely filled K and L shells. Explain.
ANSWER:-The atomic number of sodium is 11. So, neutral sodium atom
has 11 electrons and its
electronic configuration
is 2, 8,
1. But Na + has 10
electrons.
Out of 10, K-shell
contains 2
and L-shell 8 electrons
respectively. Thus, Na + has completely filled K and L shells.
10). If bromine atom is available in the form of, say, two
isotopes 79 / 35Br (49.7%) and 81 / 35Br (50.3%), calculate the average atomic
mass of bromine atom.

11) 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?
18 / 8 X be y%. Thus,
the percentage of isotope
16 /
8 X will be (100 – y) %.
Therefore,

18y + 1600 – 16y = 1620
2y + 1600 = 1620
2y = 1620 – 1600
y= 10
Therefore, the percentage
ofisotope 18 / 8 X is 10%.
And, the percentage of isotope
16 / 8 X is (100 – 10) % = 90%.
12) If Z = 3, what would be the
valency of the element? Also, name the element.
ANSWER:-By Z = 3, we mean that the atomic number of the element is
3. Its electronic
configuration is 2, 1.
Hence,
the valency of the
element is 1 (since the outermost shell
has
only one electron).
Therefore, the element with Z = 3
is lithium.
13) 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?
ANSWER:-Mass number of X = Number
of protons + Number of
neutrons
= 6 + 6
= 12
Mass number of Y = Number of
protons + Number of
neutrons
= 6 + 8
= 14
These two atomic species X and
Y have the same atomic
number, but different
mass
numbers. Hence, they are isotopes
14) 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.
False
(b) A neutron is formed by an
electron and a proton combining together. Therefore, it is neutral.
False
(c)
The mass of an electron is
that of
proton.
True
(d) An isotope of iodine is
used for making tincture iodine, which is used as a medicine.
False
15) Rutherfords alpha-particle scattering
experiment was
responsible
for the discovery
of
(a) Atomic nucleus
(b) Electron
(c) Proton
(d) Neutron
16) Isotopes of an element have
(a) The same physical
properties
(b) Different chemical
properties
(c) Different number of
neutrons
(d) Different atomic numbers
17) Number of valence electrons
in Cl – ion are:
(a) 16
(b) 8
(c) 17
(d) 18
18) Which one of the following
is a correct electronic configuration of sodium?
(a) 2, 8
(b) 8, 2, 1
(c) 2, 1, 8
(d) 2, 8, 1