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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Nuclear and Radiochemistry Page 1

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1.

Structure of an atom

 

(i) The central part, nucleus, consists of protons and neutrons.

(ii) The negatively charged extranuclear electrons revolve around the nucleus.

 

2.

Characteristics of protons, neutrons and electrons

 

 

Protons

Electrons

Neutrons

Charge

Unit positive

Unit negative

Chargeless

Mass

Nearly the same as the mass of H2 atom

1/1837 th the mass of proton or H2 atom.

Very close to the mass of H2 atom

Symbol

+

1

 P

or

+

1

 H

1

1

-

0

 e

1

1

 n

0

 

3.

Atomic number (Z)

 

The number of protons present in the nucleus of an atom of an element is called the atomic number.

 

Atomic number (Z) = Number of protons = Number of electrons

 

4.

Atomic mass number (A)

 

The total number of protons and neutrons present in the nucleus of an atom of an element is called the atomic mass number.

 

Atomic mass number (A) = Atomic number (Z) + Neutron number (N)

 

5.

Neutron number

 

The number of neutrons present in the nucleus of an atom of an isotope of an element is called the neutron number.

 

N = A - Z

 

6.

Isotopes

 

The atoms of the same element which differ in the number of neutrons present in the nuclei of their atoms are called isotopes.

Or

The atoms of the same element having the same atomic number but different atomic mass number are called isotopes.

 

e.g.

(i) Isotopes of hydrogen -  

1

 H

,

1

2

 D

,

1

3

 T

1

 

 

(ii) Isotopes of oxygen - 

16

 O

,

8

17

 O

,

8

18

 O

8

 

7.

Isobars

 

The atoms of different elements which have the same atomic mass number but different atomic numbers are called isobars.

 

e.g.

(i)  

14

 C

,

6

14

 N

7

 

 

(ii) 

40

 Ar

,

18

40

 K

,

19

40

 Ca

20

 

8.

Nuclear forces

 

  1. The strong attractive forces which exist between the protons and neutrons present in the nucleus of an atom are called nuclear forces. They are (p - n), (p - p) and (n - n) forces.
  2. They are short range forces operating in the range of 10-15.They are also called exchanged forces.
  3. The origin of nuclear forces was explained by Japnese scientist Yukawa in 1935.
  4. It is 1040 times stronger than gravitational force and 102 times stronger than the electromagnetic force.
  5. The nuclear force is due to the constant exchange of mesons (p+, p-, p) between protons and neutrons.
  6. The following exchanges take place within the nucleus, and protons and neutrons are constantly interconverted:
    p
    n + p+ ; n + p+ p
    n
    p + p - ; p + p - n
    n
    n + p ; p p + p

 

9.

Mass defect (D m)

 

The difference between the total mass of the nucleons (protons and neutrons) present in the nucleus of an atom and the actual mass of the nucleus is called the mass defect of the nucleus.

D m = [ZmH + (A - Z)mn] - M

where 

mH = mass of an atom of hydrogen isotope 

1

 H

1

 

M = isotopic mass of an atom

 

 

 

10.

Binding energy (B.E.)

 

The amount of energy required to break the nucleus of an atom into its constituent nucleons is called binding energy of the nucleus.

 

B.E. = Dm 931 MeV

 

B.E. per nucleon = 

Total B.E

=

D m 931 MeV

Mass number (A)

A

 

Binding energy affects the stability of the nucleus.

 

11.

Radioactivity

 

The phenomenon of spontaneous disintegration of unstable nuclei of certain heavy elements with the emission of some radioactive radiation is called radioactivity.
It is not affected by external factors like temperature, pressure, catalyst and the state of existence, i.e. whether it is an element or in a combined state.

 

12.

Characteristics of a, b and g - rays

 

 

a-rays

b-rays

g-rays

Charge and mass

(+ 2) charge and 4 a.m.u. mass

(-1) charge and negligible mass

Chargeless and zero mass

Origin

4

 He

 nucleus

2

Electron

Electromagnetic radiation

Velocity

1/10th to 1/100th the velocity of light

99% of the velocity of light

Same as the velocity of light

Penetrating power

Poor; can hardly pass through 0.02 cm thick Al sheet.

Greater than a-rays; can pass through 0.2 cm thick Al sheet.

Very high; can pass through 100 cm thick Al sheet.

Ionising power

High

Lower than a-rays

Very low

Deflection in an electric field

Towards the negative plate

Towards the positive plate

No deflection

 

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