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Question-1

What are canal rays?

Solution:

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 the electron. This particle is known as proton.

Question-2

If an atom contains one electron and one proton, will it carry any charge or not?

Solution:
The atom will be neutral and will not carry any charge since the magnitude of charge on an electron and proton is the same.

Question-3

On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole. 

Solution:
The negative and positive charges are equal in number and magnitude. So the 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?

Solution:
Protons reside in the nucleus of an atom on the basis of Rutherford’s model of an atom.

Question-5

Draw a sketch of Bohr’s model of an atom with three shells.

Solution:
 

 

Question-6

What do you think would be the observation if the -particle scattering experiment is carried out using a foil of a metal other than gold?

Solution:
If -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 since other metals are not so malleable and such a thin foil is difficult to obtain. If you use a thick foil, then more -particles would bounce back and no idea about the location of positive mass in the atom would be available with such certainity.

Question-7

Name the three sub-atomic particles of an atom.

Solution:
The proton, electron and neutrons are the three sub-atomic particles of an atom.

Question-8

Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

Solution:
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 X 1) u = 2 u. Then, the remaining mass (4 — 2) u = 2 u is contributed by 2 neutrons. Thus helium atom has two neutrons.

Question-9

Write the distribution of electrons in carbon and sodium atoms.

Solution:
The carbon has 6 electrons. So K-shell has 2 electrons and L-shell has 4 electrons.
Sodium has 11 electrons. The K shell has 2 electrons, L-shell has 8 electrons and M-shell has 1 electron.

Question-10

If K and L shells of an atom are full, then what would be the total number of electrons in the atom?

Solution:
K-shell can accommodate maximum 2 electrons and L- shell can accommodate 8 electrons and so the total number of electrons should be 10.

Question-11

How will you find the valency of chlorine, sulphur and magnesium?

Solution:
When the outermost shell of an atom contains 4 or more electrons, its valency is equal to 8 minus the number of electrons in the outermost shell. The outermost shell of oxygen has 6 electrons, nitrogen has 5 electrons and fluorine has 7 electrons.

The valency of oxygen is 8-6 =2

The valency of nitrogen is 8-5 =3

The valency of fluorine is 8-7= 1

Question-12

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?  

Solution:
The atomic number of the atom is 8 and charge on the atom is 0.

Question-13

With the help of Table 4.1, find out the mass number of oxygen and sulphur atom.
 


Solution:
From table,

Number of protons in an oxygen atom = 8

Number of neutrons in an oxygen atom=8

Mass number of oxygen =8+8 + 16

Number of protons in a sulphur atom = 16

Number of neutrons in a sulphur atom =16

Mass number of a sulphur atom = 16 + 16 = 32.

Question-14

For the symbol H, D and T tabulate three sub-atomic particles found in each of them.

Solution:
 
 

Hydrogen (H)

Deutrium(D)

Tritium (T)

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.

Solution:
Carbon has the isotope of and are isotopes, have the same electronic configuration as (2,6)
and are isobars. They have different electronic configurations as
is 2,8 and is 2,8,1

Question-16

Compare the properties of electrons, protons and neutrons.

Solution:
 

Particle

Nature of charge

Mass

Location

Electron

Negative

9.0 10-31 Kg

Extra nuclear part

Proton

positive

1.672 10 -27 Kg

Nucleus

Neutron

No charge

1.672 10 -27 Kg

nucleus

 

Question-17

What are the limitations of J. J. Thomson’s model of the atom?

Solution:
J. J. Thomson attributed mass of an atom due to electrons and protons which are evenly spread throughout the atom. This was not agreed  with the observations of Rutherford who concluded that mass is concentrated in a very small space later called nucleus.

Question-18

What are the limitations of Rutherford’s model of the atom?

Solution:
Charged bodies, when move in a circular motion emit radiations. Thus, electrons revolving round the nucleus, as suggested by Rutherford, will lose energy and will come closer and closer to the nucleus and will finally merge into the nucleus. This means that an atom must be unstable, which is not true.

Question-19

Describe Bohr’s model of the atom.

Solution:
 

According to Bohr’s theory:

i. The atom consists of a small positively charged nucleus at its centre.

ii. The whole mass of the atom is concentrated at the nucleus and the volume of nucleus is smaller than the volume of the atom by a ratio  of about 1:105.

iii. The nucleus contains all the protons and neutrons of the atom.

iv. The electrons of the atom revolve round the nucleus in definite circular paths known as orbits which are designated as K, L, M, N or numbered as n=1,2,3,4 outward from the nucleus.

v. Each orbit is associated with a fixed amount of energy. Therefore, these orbits are also known as energy levels or energy shells.

Question-20

Compare all the proposed models of an atom given in this chapter. 

Solution:
J. J. Thomson: Since discharge tube experiments suggested the presence of negatively charged particles in an atom which is neutral, J. J. Thomson suggested that electrons are embedded in a sphere of positive charge.

E. Rutherford: The gold foil experiments of Rutherford suggested that all the positive charge is located in a very small space which is 10-5 times the radius of an atom. Therefore, Rutherford gave a model in which electrons are revolving round the nucleus.

Neils Bohr: To explain the stability of an atom and atomic spectra, Bohr suggested that electrons are moving round the nucleus in orbits which have fixed energy shells. There is a loss or gain in energy of electron when it moves from one orbit to the other.

Question-21

Summarise the rules for the writing of distribution of electrons in various shells for the first eighteen elements.

Solution:
The below tabular column gives the distribution of electrons in various shells for the first eighteen elements.
 


Following rules are to be followed to fill electrons in different energy levels.
i. If n gives the number of orbit or energy level, then 2n2 gives the maximum number of electrons possible in a given orbit or energy level. Thus K shell can have a maximum of 2 electrons, L shell can have a maximum of 8 electrons and M-shell can have a maximum of 18 electrons.

ii. If it is the outermost orbit, then it should have more than 8 electrons.

iii. There should be stepwise filling of electrons in different orbits, i.e electrons are not accommodated in a given orbit if the earlier orbits or shells are incompletely filled.

Question-22

Define valency by taking examples of silicon and oxygen.  

Solution:
Valency is the combining capacity of an element. The number of electrons present in the outermost shell of an atom give its valency. But if the number of electrons in the outermost shell of an atom is close to its full capacity, then the valency is equal to 8 minus the number of electrons in the outermost shell.

The valency of oxygen is -2 because it can combine with two units of positive charge, Mg+2O-2 . The valency of silicon is + 4 because it can combine with 4 units of negative charge, SiCl4.

Question-23

Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and iv) Isobars. Give any two uses of isotopes. 

Solution:
i) Atomic number is defined as the number of protons present in the nucleus of an atom. For example, there are 6 protons in carbon, so the atomic number of carbon is 6. All the atoms are characterized by their atomic numbers.

ii) Mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom. For example there are 6 protons and 6 neutrons in the nucleus of carbon, so its mass number is 12.

iii) Isotopes are atoms of the same element thus having same atomic number but different mass number. For example, chlorine has two isotopes with atomic number 17 but mass numbers as 35 and 37.

iv) Isobars are such atoms which have the same mass number but different atomic numbers. Thus isobars are different elements. For example, Ne has atomic number as 10 and sodium has atomic number as 11 but both of these mass numbers are 22.
Uses of Isotopes:

1. The cobalt -60 isotope is used in the treatment of cancer.

2. The uranium-235 isotope is used as fuel in nuclear reactors.

Question-24

Na+ has completely filled K and L shells. Explain.

Solution:

The atomic number of sodium is 11. So neutral sodium atom has 11 electrons. Na+ has 10 electrons. Out of 10, K-shell contains 2 electrons and L-shell 8 electrons. Thus Na+ has completely filled K and L shells.

Question-25

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.

Solution:

Isotope of bromine with atomic mass 79 u = 49.7%

Contribution of to atomic mass = = 39.26 u

Isotope of bromine with atomic mass 81 u = 50.3%
Contribution of to the atomic mass of bromine ==
40.64 u
Hence atomic mass of the bromine atom = 39.26 + 40.64 u = 80u

Question-26

The average atomic mass of a sample of an element is 16.2 u.
What are the percentages of isotopes and in the sample?

Solution:

Let the percentage of be A

Then the percentage of = 100 –A

Then we have

+ = 16.2

A=90

 

is 90% and is 10%.

Question-27


If Z = 3, what would be the valency of the element? Also, name the element.

Solution:

The element which has atomic number as 3 is lithium. It has distribution of electrons as 2,1. Thus lithium has a valency of 1.

Question-28


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?
 

Solution:

The mass number of X = 6+6=12

The mass number of Y = 6+8=14

Since X and Y both have atomic numbers as 6 but mass numbers are different, these are isotopes to each other.

Question-29

Complete the following table.


Solution:

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

Deutrium

1

1

0

1

0

Protium

 

Question-30


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 the combination of an electron and a proton . Therefore, it is neutral.
(c) The mass of an electron is about times that of proton

(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.

Solution:

(a) False
(b) False
(c) True
(d) True

Question-31

Choose the correct choice from the questions below:

 i) Rutherford’s alpha-particle scattering experiment was responsible for the discovery of Atomic Nucleus (b) Electron (c) Proton (d) Neutron

ii). Isotopes of an element have:

(a) the same physical properties

(b) different chemical properties

(c) different number of neutrons

(d) different atomic numbers.

Solution:
Answers:
i)Atomic Nucleus.

ii) different number of neutrons.

Question-32

Choose the correct choice from the questions below:

iii. Number of valence electrons in Cl– ion are:

(a)16 (b) 8 (c) 17 (d) 18


iv. 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.

 

Solution:
Answers:
 i)8
ii)2,8,1




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