What is defined as the sum of the number of protons and neutrons in an atom's nucleus?

The sum of the number of protons and neutrons in an atom's nucleus is termed the mass number. This value provides critical information about an isotope of an element, as it reflects the total count of the two types of nucleons (protons and neutrons) that comprise the atomic nucleus. The mass number effectively indicates the overall mass of the nucleus, though it is not the same as the atomic mass, which takes into account the relative abundance of isotopes and their respective masses.

Conversely, the atomic mass often includes contributions from electrons, and the relative atomic mass is a weighted average of the masses of an element's isotopes compared to carbon-12. Nuclear charge, on the other hand, refers to the total positive charge of an atom's nucleus, determined by the number of protons, rather than the total number of nucleons. Therefore, understanding the definition of mass number as the sum of protons and neutrons is fundamental to grasping the structure of atomic nuclei and their isotopes.