Question

protons: neutrons: electrons: 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 ($p$) and neutrons ($n$), i.e., $A=p + n$.

Step2: Count protons and neutrons

From the image, there is 1 proton and 0 neutrons.

Step3: Calculate mass number

$A=1 + 0=1$.

Answer:

1