NCERT solution of class 9 chapter structure of the atom
Page no. 43
Question 1:
What are canal rays?
Answer 1:
Canal rays are positively charged radiations. These rays
consist of positively charged particles
known as protons. They were discovered by Gold stein in
1886.
Question 2:
If an atom contains one electron and one proton, will it
carry any charge or not?
Answer 2:
An electron is a negatively charged particle, whereas a
proton is a positively charged particle.
The magnitude of their charges is equal. Therefore, an atom
containing one electron and one proton
will not carry any charge. Thus, it will be a neutral atom.
Page no. 49(i)
Question 1:
On the basis of Thomson’s model of an atom, explain how the
atom is neutral as a whole.
Answer 1:
According to Thomson’s model of the atom, an atom consists
of both negatively and positively
charged particles. The negatively charged particles are
embedded in the positively charged sphere.
These negative and positive charges are equal in magnitude.
Thus, by counterbalancing each
other’s effect, they make an atom neutral.
Question 2:
On the basis of Rutherford’s model of an atom, which
subatomic particle is present in the nucleus
of an atom?
Answer 2:
On the basis of Rutherford’s model of an atom, protons
(positively-charged particles) are present
in the nucleus of an atom.
Question 3:
Draw a sketch of Bohr’s model of an atom with three shells.
Answer 3:
Question 4:
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 4:
If the α-scattering experiment is carried out using a foil
of a metal rather than gold, there would be
no change in the observation. In the α-scattering
experiment, a gold foil was taken because gold is
malleable and a thin foil of gold can be easily made. It is
difficult to make such foils from other
metals.
Page no.(ii)
Question 1:
Name the three sub-atomic particles of an atom.
Answer 1:
The three sub-atomic particles of an atom are:
(i) Protons
(ii) Electrons, and
(iii) Neutrons
Question 2:
Helium atom has an atomic mass of 4 u and two protons in its
nucleus. How many neutrons does
it have?
Answer 2:
Helium atom has two neutrons. The mass of an atom is the sum
of the masses of protons and
neutrons present in its nucleus.
Since helium atom has two protons, mass contributed by the
two protons is (2 × 1) u = 2 u.
Then, the remaining mass (4 − 2) u = 2 u is contributed by
2u /1u = 2 neutrons.
Page no. 50
Question 1:
Write the distribution of electrons in carbon and sodium
atoms?
Answer 1:
The total number of electrons in a carbon atom is 6. The
distribution of electrons in carbon atom
is given by:
First orbit or K-shell
= 2
electrons
Second orbit or
L-shell = 4
electrons
Or,
we can write the distribution of electrons in a carbon atom
as 2, 4.
The total number of electrons in a sodium atom is 11. The
distribution of electrons in sodium atom
is given by:
First orbit or
K-shell = 2
electrons
Second orbit or L-shell = 8 electrons
Third orbit or M-shell = 1 electron
Or,
we can write distribution of electrons in a sodium atom as
2, 8, 1.
Question 2:
If K and L shells of an atom are full, then what would be
the total number of electrons in the atom?
Answer 2:
The maximum number of electrons that can occupy K and
L-shells of an atom are 2 and 8
respectively. Therefore, if K and L-shells of an atom are
full, then the total number of electrons in
the atom would be (2 + 8) = 10 electrons.
Page no. 52(i)
Question 1:
How will you find the valency of chlorine, sulphur and
magnesium?
Answer 1:
If the number of electrons in the outermost shell of the
atom of an element is less than or equal to
4, then the valency of the element is equal to the number of
electrons in the outermost shell.
On the other hand, if the number of electrons in the outermost
shell of the atom of an element is
greater than 4, then the valency of that element is
determined by subtracting the number of
electrons in the outermost shell from 8.
The distribution of electrons in chlorine, sulphur, and
magnesium atoms are 2, 8, 7; 2, 8, 6 and 2,
8, 2 respectively. Therefore, the number of electrons in the
outer most shell of chlorine, sulphur,
and magnesium atoms are 7, 6, and 2 respectively.
Thus,
The valency of chlorine = 8 −7 = 1
The valency of sulphur = 8 − 6 = 2
The valency of magnesium = 2
Page no. 52(ii)
Question 1:
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 1:
(i) The atomic number is equal to the number of protons.
Therefore, the atomic number of the atom is 8.
(ii) Since the number of both electrons and protons is equal,
therefore, the charge on the atom is 0.
Question 2:
With the help of Table 4.1, find out the mass number of
oxygen and sulphur atom.
Answer 2:
Mass number of oxygen = Number of protons + Number of
neutrons = 8 + 8 = 16
Mass number of sulphur = Number of protons + Number of
neutrons = 16 +16 = 32
page no. 53
Question 2:
Write the electronic configuration of any pair
of isotopes and isobars.
Ans.
Exercise
Question 1:
Compare the properties of electrons, protons and neutrons.
Question 2:
What are the limitations of J.J. Thomson’s model of the
atom?
Answer 2:
According to J.J. Thomson’s model of an atom, an atom
consists of a positively charged sphere with electrons embedded in it. However,
it was later found that the positively charged particles reside at the center
of the atom called the nucleus, and the electrons revolve around the nucleus.
Question 3:
What are the limitations of Rutherford’s model of the atom?
Answer 3:
According to Rutherford’s model of an atom, electrons
revolve around the nucleus in fixed orbits. But, an electron revolving in circular
orbits will not be stable because during revolution, it will experience
acceleration. Due to acceleration, the electrons will lose energy in the form
of radiation and fall into the nucleus. In such a case, the atom would be
highly unstable and collapse.
Question 4:
Describe Bohr’s model of the
atom.
Answer 4:
Bohr’s model of the atom Niels
Bohr proposed the following postulates regarding the model of the atom.
(i) Only certain orbits known as discrete orbits of
electrons are allowed inside the atom. (ii) While revolving in these
discrete orbits, the electrons do not radiate energy. These discrete orbits or
shells are shown in the following diagram.
The first orbit (i.e., for n = 1) is represented by letter
K. Similarly, for n = 2, it is L − shell, for n = 3, it is M − shell and for n
= 4, it is N − shell. These orbits or shells are also called energy levels.
Question 6:
Summarize the rules for writing of distribution of electrons
in various shells for the first eighteen elements.
Answer 6:
The rules for writing of the distribution of electrons in
various shells for the first eighteen elements are given below:
(i) The maximum number of electrons that a shell can
accommodate is given by the formula '2n2', where 'n' is the orbit number or
energy level index (n = 1, 2, 3...).
The maximum number of electrons present in an orbit of n = 1
given by 2n2 = 2 × 12 = 2 Similarly, for second orbit, it is 2n2 = 2 × 22 = 8
For third orbit, it is 2n2 = 2 × 32 = 18 And so on ..... (ii) The outermost
orbit can be accommodated by a maximum number of 8 electrons.
(iii) Shells are filled with electrons in a step wise manner
i.e., the outer shell is not occupied with electrons unless the inner shells
are completely filled with electrons.
Question 7:
Define valency by taking examples of silicon and oxygen.
Answer 7:
The valency of an element is the combining capacity of that
element. The valency of an element is determined by the number of valence
electrons present in the atom of that element. If the number of valence
electrons of the atom of an element is less than or equal to four, then the
valency of that element is equal to the number of valence electrons. For
example, the atom of silicon has four valence electrons. Thus, the valency of
silicon is four. On the other hand, if the number of valence electrons of the
atom of an element is greater than four, then the valency of that element is
obtained by subtracting the number of valence electrons from eight. For
example, the atom of oxygen has six valence electrons. Thus, the valency of
oxygen is (8 − 6) i.e., two.
Question 8:
Explain with examples
(i) Atomic number,
(ii) Mass number,
(iii) Isotopes and
(iv) Isobars.
Give any two uses of isotopes.
Answer 8: (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
They are atoms of the same element and have same atomic
number but different mass number/atomic mass. For example:
Carbon: 𝐶612 and 𝐶614
(iv) Isobars
They are atoms of different elements having same mass number
but different atomic number.
For example calcium, atomic number 20 and argon, atomic
number 18.
The number of electrons in these atoms is different, but the
mass number of both these elements is 40. That is, the total number of neutrons
is the same in the atoms of this pair of elements.
Two uses of isotopes are as follows:
(i) An isotope of uranium is used as a fuel in nuclear reactors.
(ii) An isotope of cobalt is used in the treatment of
cancer.
Question 9:
Na+ has completely filled K and L shells. Explain.
Answer 9:
Na has atomic number 11, so its electronic configuration is
= 2, 8, 1
When it gives away its outermost shell single electron it
changes to Na+ =10= 2, 8
The above configuration indicates completely filled K, L
shells.
Question 10:
If bromine atom is available in the form of, say, two
isotopes 𝐵𝑟3579 (49.7%) and 𝐵𝑟3581
(50.3%), calculate the average atomic mass of bromine atom.
Answer 10:
The average atomic mass of bromine
= (79 x 49.7)+ (81 x 50.3)/100
= (3926.3 + 4074.3)/100
= 8000.6/100
= 80 u
Question 11:
Answer 11:
Since average atomic mass
= 16 x X + 18 x (100 – X )/100
16.2 = 16X + 1800 – 18X/100
1620 = – 2X + 1800
2X = 1800 – 1620
X = 180/2 = 90
When 90% is the X-16 sample so for X-18 sample % =
100-90=10%
Question 12:
If Z = 3, what would be the valency of the element? Also,
name the element.
Answer 12:
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.
Question 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 13:
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.
Question 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.
(b) A neutron is formed by an electron and a proton
combining together. Therefore, it is neutral.
(c) The mass of an electron is about 1/2000 times that of
proton.
(d) An isotope of iodine is used for making tincture iodine,
which is used as a medicine.
Answer 14:
(a) J.J. Thomson proposed that the nucleus of an atom
contains only nucleons. (F)
(b) A neutron is formed by an electron and a proton
combining together. Therefore, it is neutral. (F)
(c) The mass of an electron is about 1/2000 times that of
proton. (T)
(d) An isotope of iodine is used for making tincture iodine,
which is used as a medicine. (T)
Question 15:
Put tick (√ ) against correct choice and cross ( x ) against
wrong choice in the following question: Rutherford’s alpha-particle scattering
experiment was responsible for the discovery of
(a) Atomic nucleus
(b) Electron
(c) Proton
(d) Neutron
Answer 15:
Rutherford’s alpha-particle scattering experiment was
responsible for the discovery of
(a) Atomic nucleus (√ )
(b) Electron ( x )
(c) Proton ( x )
(d) Neutron ( x )
Question 16:
Put tick ( √ ) against correct choice and cross ( x )
against wrong choice in the following question: Isotopes of an element have
(a) the same physical properties
(b) different chemical properties
(c) different number of neutrons
(d) different atomic numbers
Answer 16:
Isotopes of an element have
(a) the same physical properties ( x )
(b) different chemical properties ( x )
(c) different number of neutrons (√ )
(d) different atomic numbers ( x )
Question 17:
Put tick (√ ) against correct choice and cross ( x ) against
wrong choice in the following question: Number of valence electrons in Cl− ion
are:
(a) 16
(b) 8
(c) 17
(d) 18
Answer 17:
Number of valence electrons in Cl− ion are:
(a) 16 ( x )
(b) 8 (√ )
(c) 17 ( x )
(d) 18 ( x )
Question 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
Answer 18:
(d) The correct electronic configuration of sodium is 2, 8,
1.
