STRUCTURE OF ATOM
1. What are canal rays?
Canal
rays are positively charged radiations. These rays consist of positively
charged particles known as protons. They were discovered by Goldstein in 1886.
2. If an atom contains one electron and
one proton, will it carry any charge or not?
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.
3. On the basis of Thomson’s model of
an atom, explain how the atom is neutral as a whole.
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.
4. On the basis of Rutherford’s model
of an atom, which subatomic particle is present in the nucleus of an atom?
On
the basis of Rutherford's model of an atom, protons (positively-charged particles)
are present in the nucleus of an atom.
5. Draw a sketch of Bohr’s model of an
atom with three shells.
Bohr’s
model of an atom with three shells
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?
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.
7. Name the three sub-atomic particles
of an atom.
The
three sub-atomic particles of an atom are:
(i) Protons
(ii) Electrons, and
(iii) Neutrons
8. Helium atom has an atomic mass of 4
u and two protons in its nucleus. How many neutrons does it have?
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 
neutrons.
neutrons.
9. Write the distribution of electrons
in carbon and sodium atoms?
Thetotal
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.
10. If K and L shells of an atom are
full, then what would be the total number of electrons in the atom?
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.
11. How will you find the valency of
chlorine, sulphur and magnesium?
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
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?
(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.
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
For
the symbol H, D and T tabulate three sub-atomic particles found in each of
them.
Symbol
|
Proton
|
Neutron
|
Electron
|
H
|
1
|
0
|
1
|
D
|
1
|
1
|
1
|
T
|
1
|
2
|
1
|
Write
the electronic configuration of any one pair of isotopes and isobars.
Two
isotopes of carbon are 
and
.
and.
The
electronic configuration of is 2, 4.
is 2, 4.
The
electronic configuration of is 2, 4.
is 2, 4.
[Isotopes
have the same electronic configuration]
and 
are a pair of isobars
The
electronic configuration of is 2, 8, 8, 2.
is 2, 8, 8, 2.
The
electronic configuration of is 2, 8, 8.
is 2, 8, 8.
Question 1:
Compare
the properties of electrons, protons and neutrons.
Electron
|
Proton
|
Neutron
|
|||
(i)
|
Electrons
are present outside the nucleus of an atom.
|
(i)
|
Protons
are present in the nucleus of an atom.
|
(i)
|
Neutrons
are present in the nucleus of an atom.
|
(ii)
|
Electrons
are negatively charged.
|
(ii)
|
Protons
are positively charged.
|
(ii)
|
Neutrons
are neutral.
|
(iii)
|
The
mass of an electron is considered to negligible.
|
(iii)
|
The
mass of a proton is approximately 2000 times as the mass of an electron.
|
(iii)
|
The
mass of neutron is nearly equal to the mass of a proton.
|
What
are the limitations of J.J. Thomson’s model of the atom?
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 centre of the atom called the
nucleus, and the electrons revolve around the nucleus.
What
are the limitations of Rutherford’s model of the atom?
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.
Describe
Bohr’s model of the atom.
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.
Compare
all the proposed models of an atom given in this chapter.
·
|
Summarize
the rules for writing of distribution of electrons in various shells for the
first eighteen elements.
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 is 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 stepwise manner i.e., the outer shell is
not occupied with electrons unless the inner shells are completely filled with
electrons.
Define
valency by taking examples of silicon and oxygen.
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.
(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
Isotopes
are atoms of the same element having the same atomic number, but different mass
numbers. For example, hydrogen has three isotopes. They are protium
, deuterium,
and tritium
.
, deuterium,and tritium.
(iv)
Isobars
Isobars
are atoms having the same mass number, but different atomic numbers i.e., isobars
are atoms of different elements having the same mass number. For example, 
and
are isobars.
andare isobars.
Two
uses of isotopes are:
(i)
One isotope of uranium is used as a fuel in nuclear reactors.
(ii)
One isotope of cobalt is used in the treatment of cancer.
Question 9:
Na+
has completely filled K and L shells. Explain.
An
atom of Na has a total of 11 electrons. Its electronic configuration is 2, 8,
1. But, Na+ ion has one electron less than Na atom i.e., it has 10
electrons. Therefore, 2 electrons go to K-shell and 8 electrons go to L-shell,
thereby completely filling K and L shells.
Question 10:
If
bromine atom is available in the form of, say, two isotopes 
(49.7%) and 
(50.3%), calculate the average
atomic mass of bromine atom.
(49.7%) and (50.3%), calculate the average
atomic mass of bromine atom.
It
is given that two isotopes of bromine are (49.7%) and 
(50.3%). Then, the average atomic
mass of bromine atom is given by:
(49.7%) and (50.3%). Then, the average atomic
mass of bromine atom is given by: 
Question 11:
The
average atomic mass of a sample of an element X is 16.2 u. What are the
percentages of isotopes 
and 
in the sample?
and in the sample?
It
is given that the average atomic mass of the sample of element X is 16.2 u.
Let
the percentage of isotope be y%. Thus, the percentage
of isotope will be (100 − y) %.
be y%. Thus, the percentage
of isotope will be (100 − y) %.
Therefore,
Therefore,
the percentage of isotopeis 10%.
is 10%.
And,
the percentage of isotopeis (100 − 10) % = 90%.
is (100 − 10) % = 90%.
If
Z = 3, what would be the valency of the element? Also, name the element.
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.
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?
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 
times that of proton.
times that of proton.
(d)
An isotope of iodine is used for making tincture iodine, which is used as a
medicine.
(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 times that of proton. (T)
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 (
) against wrong choice in the
following question:
) against correct choice and cross () 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
Rutherford’s
alpha-particle scattering experiment was responsible for the discovery of
(a)
Atomic nucleus
|
 |
(b)
Electron
|
 |
(c)
Proton
|
|
(d)
Neutron
|
|
Question 16:
Put
tick () against correct choice and cross () against wrong choice in the
following question:
) against correct choice and cross () 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
Isotopes
of an element have
(a)
the same physical properties
|
|
(b)
different chemical properties
|
|
(c)
different number of neutrons
|
|
(d)
different atomic numbers
|
|
Put
tick () against correct choice and cross () against wrong choice in the
following question:
) against correct choice and cross () against wrong choice in the
following question:
Number
of valence electrons in Cl− ion are:
(a)
16
(b)
8
(c)
17
(d)
18
Number
of valence electrons in Cl− ion are:
(a) 16
|
|
(b) 8
|
|
(c) 17
|
|
(d) 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
(d) The correct electronic configuration of
sodium is 2, 8, 1.
Question 19:
Complete
the following table.
Atomic number
|
Mass number
|
Number of Neutrons
|
Number of protons
|
Number of electrons
|
Name of the Atomic species
|
9
|
−
|
10
|
−
|
−
|
−
|
16
|
32
|
−
|
−
|
−
|
Sulphur
|
−
|
24
|
−
|
12
|
−
|
−
|
−
|
2
|
−
|
1
|
−
|
−
|
−
|
1
|
0
|
1
|
1
|
−
|
·
Answer
Atomic number
|
Mass number
|
Number of Neutrons
|
Number of protons
|
Number of electrons
|
Name of the Atomic species
|
9
|
19
|
10
|
9
|
9
|
Fluorine
|
16
|
32
|
16
|
16
|
16
|
Sulphur
|
12
|
24
|
12
|
12
|
12
|
Magnesium
|
1
|
2
|
1
|
1
|
1
|
Deuterium
|
1
|
1
|
0
|
1
|
1
|
Protium
|
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