Chapter 4,
Structure of the Atom
Class 9, NCERT (CBSE) Science - Solutions of In-Text Questions
NCERT Solutions - CBSE Guide
Question.1: What are canal rays?
Answer:
Canal rays are positively charged radiations consisting of particles which have
a charge equal in magnitude but opposite in sign to that of the electron. The
mass of a canal ray particle is 2000 times as that of an electron. This
particle is known as proton. Canal rays were discovered by Goldstein.
Question.2: If an atom contains one
electron and one proton, will it carry any charge or not?
Answer:
Since the magnitude of the charge on an electron and proton is the same, the
atom containing only one electron and one proton will carry no charge and will
be electrically neutral.
Question.3: On the basis of Thomson’s model of an atom,
explain how the atom is neutral as a whole.
Answer:
As per Thomson’s model of an atom the negative and positive charges are equal
in number and magnitude. So, they balance each other as a result of which atom
as a whole is electrically neutral.
Question.4: On the basis of Rutherford’s model of an atom,
which subatomic particle is present in the nucleus of an atom?
Answer:
According to the Rutherford’s model of an atom, protons reside in the nucleus
of an atom.
Question.5: Draw a sketch of Bohr’s Model of an atom with
three shells.
Answer:
Refer to NCERT textbook.
Question.7: 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:
If α-particle scattering experiment is carried out using a foil of any metal as
thin as gold foil used by Rutherford, there would be no change in observations.
But since other metals are not so malleable so, such a thin foil is difficult
to obtain. If we use a thick foil, then more α-particles would bounce back and
no idea about the location of positive mass in the atom would be available with
such a certainty.
Question.8: Name the three
sub-atomic particles of an atom.
Answer:
Electron (0e–1), Proton (1p+1) and
Neutron (1n0).
Question.9: Helium atom has an atomic mass of 4u and two
protons in its nucleus. How many neutrons does it have?
Answer:
The mass of an atom is given by the sum of the masses of protons and neutrons
present in the nucleus. Since helium atom has an atomic mass of 4u and it has
two protons, the protons contribute 2u to atomic mass. Hence, it must contain 2
neutrons as each neutron contributes 1u to the nucleus.
Question.10: Write the distribution of electrons in carbon
and sodium atoms.
Answer:
Carbon has 6 electrons with electronic configuration as given below -
K-shell
– 2 electrons and L-shell – 4 electrons. That is 2, 4.
Sodium
has 11 electrons with electronic configuration as given below -
K-shell
– 2 electrons, L-shell – 8 electrons and M-shell – 1 electron. That is 2, 8, 1.
Question.11: 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 accommodate maximum 2 electrons and L-shell can accommodate 8
electrons and so, 10 is the total number of electrons provided K and L shells
are full.
Question.12: How will you find the valency of chlorine,
sulphur and magnesium?
Answer:
Electronic configurations:
K-Shell
|
L-Shell
|
M-Shell
|
|
Chlorine
|
2
|
8
|
7
|
Sulphur
|
2
|
8
|
6
|
Magnesium
|
2
|
8
|
2
|
Hence,
valency of Chlorine = 8 – 7 = 1, valency of Sulphur = 8 – 6 = 2, valency of
Magnesium = 2
Question.13: If the number of electrons in an atom is 8 and
the number of protons are also 8, then (i) what is the atomic number of the
atom? (ii) What is the charge on the atom?
Answer:
(i) Atomic number = Number of Protons = 8. (ii) Charge on the Atom = 0.
Question.14: For the symbol H, D and T tabulate three
subatomic particles found in each of them.
Ans:
Hydrogen (H) 1H1
|
Deuterium (D) 2H1
|
Tritium (T) 3H1
|
|
Electron
|
1
|
1
|
1
|
Proton
|
1
|
1
|
1
|
Neutron
|
0
|
1
|
2
|
Question.15: Write the electronic configuration of any one
pair of isotopes and isobars.
Answer:
12C6 and 14C6 are isotopes, have
the same electronic configuration as (2, 4). 22Ne10 and 22Ne11
are isobars. They have different electronic configuration as given below:
22Ne10
– 2, 8
22Ne11
– 2, 8, 1
Further Study
very important
ReplyDeleteyeah it is very important
ReplyDelete