Here's why:
* Protons and neutrons reside in the nucleus of an atom and have approximately the same mass (slightly more for neutrons).
* Electrons, which orbit the nucleus, are much smaller and have a negligible mass compared to protons and neutrons.
Therefore, the mass of an atom is mostly determined by the number of protons and neutrons it contains. This is why we often use the term "atomic mass number" which is the total number of protons and neutrons in an atom.
