Answer:
D) Decreases by 4
Explanation:
The mass number of a nucleus is equal to the number of protons and neutrons in the nucleus:
A = p + n
where
p is the number of protons
n is the number of neutrons
An alpha particle is a nucleus of helium, consisting of 2 protons and 2 neutrons. This means that when an unstable nucleus emits an alpha particle, it loses 2 protons and 2 neutrons. Therefore, the new mass number of the nucleus will be
A' = (p-2) + (n-2) = p + n - 4 = A - 4
So, it will decrease by 4 units.
