Chapter 19, Problem 53A

Interpretation Introduction

Interpretation: The process in which neutron to proton ratio decreases needs to be explained.

Concept introduction: The ratio of its number of the neutron to its number of protons is called n/p ratio of an atomic nucleus.

n/p ratio indicates the stability of the nucleus. A nuclide is most stable when n/p ratio is equal to 1 (when number of neutrons and number of protons are same). If n/p ratio is greater or lesser than 1 then radioactive nuclide decays by various radioactive decay process to approach a value which closer to one.

Number of neutron $=$ mass number − number of protons

Answer to Problem 53A

Only option “a” is correct.

Explanation of Solution

A beta decay reaction can be represented as: $A\to B+e$

Where $Z=$ number of protons

  $X=$ mass number

A obvious from the reaction that in beta emission proton number increases but number of neutron decreases and hence n/p ratio also decreases.

Conclusion

An alpha decay reaction can be represented as: $A\to B+He$

Heavier nucleus having higher number of neutrons decays by alpha emission.

X and Z are both reduced by 2. Since for nuclei that can decay by alpha emission there are always more neutrons in a nucleus than protons overall result is decrease of n/p value.

A positron decay reaction can be represented as: $A\to B+e$

Positron decay occurs in nuclide having low n/p ratio. Since one proton decreases in positron emission overall result is that n/p ratio increases.

An electron capture reaction can be represented as: $A+e\to B$

Here also n/p ratio increases.

In gamma decay neither mass not proton number changes and hence n/p ratio remains same.