Question

Heavy, unstable nuclei usually decay by emitting an $\mathit{\alpha }$or a$\mathit{\beta }$ particle. Why don’t they usually emit a single proton or neutron?

Short answer

For reducing the mass of the system, it wants to emit a particle that has a low rest mass per nucleon. A proton or neutron has more rest mass per nucleon as compared to the $\alpha -$particle.

Step-by-step solution

Define the atomic number and mass number.

The number of protons or the number of electrons present in the element is known as the atomic number $Z$.

The sum of the number of protons and the number of neutrons present in an element is known as the mass number $A$.

Explanation.

The heavy nucleus is attempting to reduce the mass. In general, any $\beta$decay takes priority to reach the valley of beta stability. For reducing the mass of the system, it wants to emit a particle that has a low rest mass per nucleon.

A proton or neutron has more rest mass per nucleon as compared to the $\alpha -$particle. ${}_4{}^{2}He$is the ideal particle that emits an equal number of protons and neutrons.