what is the mass number? 0

Explanation:

Step1: Recall mass - number formula

The mass number ($A$) of an atom is the sum of the number of protons and neutrons in the nucleus.

Step2: Count protons and neutrons

From the image, assume gray spheres are protons and red spheres are neutrons. Counting the spheres in the nucleus, if there are 2 protons and 2 neutrons. $A=\text{Number of protons}+\text{Number of neutrons}=2 + 2$

Step3: Calculate mass number

$A = 4$

Answer:

4