Question

What is a rule for determining the mass number of an atom?

Answer 1

Mass number is the sum of the protons and neutrons in the atomic nucleus.

Explanation:

The mass number (`"A"`) is the sum of the atomic number (`"Z"`), which is the number of protons, and the number of neutrons (`"N"`) in the atomic nucleus of an isotope of an element. `"A=Z+N"`