Question

(a) Rank the following decays in order of the change in atomic number Z that occurs in the decaying nucleus, from most positive (greatest increase in Z) to most negative (greatest decrease in Z). If any two decays cause the same change in Z, state this. Explain how you made your ranking.

(i) Alpha decay (emission of a 4He nucleus of charge +2e)

(ii) Beta-minus decay (emission of an electron of charge –e and an antineutrino)

(iii) Beta-plus decay (emission of a positron of charge +e and a neutrino)

(iv) Gamma decay (emission of a photon)

(b) Fill in the blanks with the phrase alpha decay, the phrase beta-minus decay, the phrase beta-plus-decay, the phrase gamma decay, or the phrase don’t know.

(i) If a nucleus is in an excited state, one common way for it to return to its ground state while retaining the same number of protons and neutrons is via __________.

(ii) If a nucleus can increase its stability by converting one proton into a neutron, it will undergo __________.

(iii) If a nucleus can increase its stability by converting one neutron into a proton, it will undergo __________.

(iv) Very large nuclei with more than 82 protons, such as radon, radium, and thorium, can increase their stability by becoming smaller. One common way in which this can happen is __________.

Answer 1

Part a. If X is the parent nucleus having atomic no. Z and Mass number A, then

2x () -decay Alpha decry) A-4 4 + He sta 2-2 ex 2 Alpha Particle (11) ß decay ( Beta minus decay) А bit té tre z Z+ (111) ßt decay (Beta flus decay) tet + ve 274 2-1 7XA (iv) or decay (Gamma decay) A X*A tipongy xx z excited state ground state

It is clear that, in case of alpha decay, the atomic no. decreases by two; in case of beta minus decay, the atomic number increases by one; in beta plus decay, the atomic number decreaes by one, however, in case of gamma emission, there is no change in the atomic number as it involves only the emission of energy to bring the nuclei from excited to ground state.

Since

Z+1>Z>Z-1>Z-2

Hence, the order of atomic number would be

Beta minus decay> Gamma decay> Beta plus decay> Alpha decay

i.e. with beta minus as the most positive while alpha decay being the most negative.

Part b. It can be clearly seen from the equations mentioned above that

1.When the nucleus is left in excited state after alpha or beta decay, it emits gamma ray(which is a neutral particle) in order to come back to ground state and be stable. Hence, in order to have same no of protons and neutrons, the nuclei returns to its ground state via Gamma Decay.

2. There is comversion of one proton to neutron in case of Beta plus decay. Hence, beta plus decay is the correct answer.

3. There is conversion of one neutron into proton in case of Beta minus decay. Hence, Beta minus decay is the correct answer.

4. In case of alpha decay, there is decrease in atomic number of daughter nuclei by 2 and decrease in mass number by 4 i.e. it gives rise to daughter nuclei having smaller size than that of the parent nuclei.

Hence, common way of attaining stability is possible by becoming a smaller nuclei via Alpha Decay.