MHT-CET : Physics Entrance Exam

MHT - CET : Physics - Atoms Molecules and Nuclei Page 5

‹‹ Previous  |  Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |   Next ››

7.

• Radioactivity is the phenomenon of spontaneous disintegration of an unstable nucleus
(
Z > 83) where a  or b particles are emitted, accompanied by g rays.

Properties of a, b particles and g rays:

 Property a b g 1. Charge +2e - e 0 2. Mass - 4mp where mp = mass of proton me where me = mass of electron Radiation 3. Speed when emitted 107 m/s Upto 99% of c c 4. Ionization of gas/air Intense Moderate Low 5. Penetrating Power Least Moderate Highest 6. Deflection by Deflected Deflected Not deflected 7. Change in Z, A of emitting nucleus A ® A - 4 Z ® Z - 2 Z ® Z + 1 No change 8. Photographic plate Affect Affect Affect 9. Produce fluorescence? Yes Yes Yes

c : speed of light

Rutherford-Soddy Laws or Group Displacement Laws:

1. When a particle is emitted, ZXA ® Z - 2YA - 4 + 2He 4
2. When b particle is emitted, ZXA® Z + 1YA + - 1e0
3. When g  rays are emitted, ZXA ® ZXA

8.

Statement: The rate of radioactive disintegration at any instant is directly proportional to the number of radioactive nuclei of the parent element present at that time.

 dN a  N dt

 \ dN = - lN dt

where l is a constant called the decay constant. Negative sign indicates that as time increases, no. of atoms decreases.

 Integrating the equation dN = - lN, we get, dt

N = N0 e -lt

This expression is called exponential law of radioactive decay.

Graph:

This graph shows the variation of the number of radioactive atoms (N) with time t.
Further, it shows that law of decay is exponential in nature.

Half Life

Half life (T) of a radioactive element is defined as the time required for half the initial number of radioactive nuclei of the element to disintegrate.

 i.e. N = N0 at t = T (half life) 2

Using this in the equation N = N0 e -lt we get,

 T = 0.693 l

t = 0 No. of atoms = N0 = N0/20

t = T

 = N0 = N0/21 2

t = 2T

 = N0 = N0/22 4

t = 3T

 = N0 = N0/23 8

\ t = nT

 N = N0 2n

 \ N0 = 2n , when t = nT N

 Decay Constant l = - dN/ N dt

Hence the decay constant l can be defined as the fraction of radioactive nuclei (dN/ N) disintegrating per unit time. The SI unit of l is (second)-1.

l is a measure of stability of a radioactive material.

‹‹ Previous  |  Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |   Next ››

 Career in India | Jobs in India