What is its mass number after the beta decay?

Question

A monoatomic atom or ion with 37 protons, 32 neutrons, and 38 electrons undergoes a beta decay. What is its mass number after the beta decay?

Answer 1

Beta decay will not affect the mass number. The mass number is the sum of the number of protons + the number of neutrons. In this case that will be 37 + 32 = 69 amu. This will be the mass number after beta decay.