Question

if an atom of sodium combines with (captures) an alpha particle that has 2 protons and 2 neutrons, its new mass number will be

Explanation:

Step1: Determine sodium's mass number

The mass number of sodium ($Na$) is $23$ (common knowledge in chemistry).

Step2: Calculate new mass number

An alpha particle has $2$ protons and $2$ neutrons. The mass number contribution of the alpha particle is $2 + 2=4$.
The new mass number is $23 + 4=25$.

Answer:

C. 25 amu