When an atomic nucleus emits a gamma ray, by how much does the mass number of the nucleus change?
Say that we have here a big atomic nucleus, a collection of protons and neutrons. We’re told that this nucleus emits a gamma ray. Now, a gamma ray is electromagnetic radiation. These rays have very high energy levels, in fact their higher energy even than X-rays. But just like X-rays as well as other parts of the electromagnetic spectrum, gamma rays are light rays. And we know that while light does have momentum, it’s not considered to have any mass. That affects our answer to this question of how this gamma ray emission affects the mass number of the nucleus that emitted it.
A nuclear mass number is equal to the sum of two types of objects, protons and neutrons. This means that the only way to change a nuclear mass number is to change its number of protons and/or its number of neutrons. But as we’ve seen, since gamma rays are light and have no mass, they don’t affect this number of protons or neutrons in the nucleus. This means that the gamma ray emission has no impact on the mass number of the nucleus. Emitting something which has no mass doesn’t change the mass of the nucleus.