The radioactive isotope plutonium-244 becomes radon-220 after a succession of decays. The atomic number of plutonium is 94 and the atomic number of radon is 86. How many alpha particles and how many beta particles are emitted altogether during the decay of one nucleus of plutonium-244 to radon-220?

Answer:

23) B

Mass number only decreases by alpha decay. After each alpha decay, the mass number decreases by 4.

Decrease in mass number = 244 – 220 = 24
Number of alpha decays = 24 ÷ 4 = 6

After 6 alpha decays, the atomic number will decrease by 12.
So atomic number would decrease to 82 (94 – 12 = 82)

After each beta decay, the atomic number increases by 1. We know the atomic number of radon is 86. So there must be 4 beta decays to increase the atomic number from 82 to 86.

After 6 alpha decays and 4 beta decays, plutonium-244 becomes radon-220.

So the answer is B.