## Nuclei  Important Questions for CBSE Class 12 Physics Radioactivity and Decay Law

1.In every atom, the positive charge and mass are densely concentrated at the centre of the atom forming its nucleus. More than 99.9% mass of the atom is concentrated in the nucleus.         ### 1 Mark Questions Ans.Neutrinos are difficult to detect because they are massless, have no charge and do not interact with nucleons.

2.Define the activity of a given radioactive substance. Write its SI unit. [All India 2013]
Ans.The activity of a sample is defined as the rate of disintegration taking place in the sample of radioactive substance.
SI unit of activity is Becquerel (Bq).
1 Bq =1 disintegration/second

3.Write any two characteristic properties  of nuclear force. [All India 2011]
Ans.Two characteristics of nuclear force are given as below:
(i) These are short range forces.
(ii) These are strong force of attractive nature

4.How is the mean life of a radioactive sample related to its half-life? [Foreign 2011]
Ans.  5.How is the radius of a nucleus related to  its mass number?  [All India 2011 c]
Ans.  Ans.  Ans. 8.Two nuclei have mass numbers in the  ratio 1: 8. What is the ratio of their nuclear radii? [All India 2009]
Ans. 9.Two nuclei have mass numbers in the  ratio 8:125. What is the ratio of their nuclear radii? [All India 2009]
Ans.Refer to ans. 8. (Ans. 2 :5)

10.Two nuclei have mass numbers in the  ratio 27 :125. What is the ratio of their nuclear radii?             [All India 2009]
Ans.Refer to ans. 8. (Ans. 3:5)

11.Two nuclei have mass numbers in the  ratio 1:2. What is the ratio of their nuclear  densities? [Delhi 2009]
Ans.Nuclear density is independent of mass number.

12.Assuming the nuclei to be spherical in shape, how does the surface area of a nucleus of mass number A1  compare with that of a nucleus of mass number A2 ?   [All India 2008 C]
Ans. 13.Out of the two characteristics, the  mass number A and the atomic number Z of a nucleus, which one does not change during nuclear decay?   [All India 2008 C]
Ans. ### 2 Marks Questions Ans.   Ans.  16.How the size of a nucleus is experimentally determined? Write the relation between the radius and mass number of the nucleus. Show that the density of nucleus is independent of its mass number.
[Delhi, 2012,2011 C]
Ans.   Ans. Ans. 19.Draw a plot representing the law of  radioactive decay . Define the activity of a sample of a radioactive nucleus. Write its SI unit.         [Foreign 2008]
Ans.   Ans. 3 Marks Questions Ans.     Ans.      Ans.  Ans.   Ans.    Ans.(i)  (ii)     Ans.   Ans.         Ans.   