How is atomic mass defined?

Atomic mass is defined as the total mass of an atom, which primarily comes from the sum of the masses of its protons and neutrons. These two subatomic particles reside in the nucleus and have significant mass relative to electrons, which contribute negligibly to the overall mass of the atom. In most typical contexts, atomic mass is expressed in atomic mass units (amu), where each proton and neutron is approximately one amu.

Thus, the correct option reflects that atomic mass is determined by the combined count of protons and neutrons present in the nucleus, making it a critical factor in defining the identity and properties of an atom. This definition allows for understanding isotopes, which are variations of elements with different neutron counts, further illustrating the importance of both protons and neutrons in defining atomic mass.