When an alpha particle is emitted from an unstable nucleus, the atomic mass numberof the nucleus A) increases by 4. B) decreases by 2. C) increases by 2. D) decreases by 4.

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.