What do we call the total number of protons plus neutrons in an atom?

The total number of protons plus neutrons in an atom is referred to as the mass number. This value is crucial in understanding an atom's identity and stability. Protons and neutrons are collectively known as nucleons, and their quantities define not only the specific isotope of an element but also affect its nuclear properties. The mass number is always a whole number since it counts discrete particles.

In contrast, atomic mass typically refers to the weighted average of the isotopes of an element and takes into account the relative abundance of each isotope. An atomic unit is more broadly defined as a unit of mass used in atomic physics, and nuclear count might seem relevant but is not a standard term in chemistry or physics. Therefore, mass number is the precise term used in this context.