What does the mass number indicate?

The mass number is defined as the total count of protons and neutrons present in the nucleus of an atom. This combined total is important because it determines the overall mass of the atom, which is why it is called the mass number.

Protons contribute one unit of mass and neutrons also contribute approximately one unit of mass, while electrons have negligible mass compared to protons and neutrons. Therefore, when you add together the number of protons (which defines the element) and the number of neutrons (which can vary among isotopes of that element), you arrive at the mass number.

Thus, the mass number reflects both the identity of the atom and its isotopic variations, making it a crucial concept in understanding atomic structure and behavior.