Final answer:
The overall mass of an atom is nearly equal to its mass number, which is the sum of the number of protons and neutrons, as both protons and neutrons have roughly the same mass of about 1 amu each.
Step-by-step explanation:
The overall mass of an atom is best described by the number of protons and neutrons it contains. The mass number (A) of an atom is the sum of the number of protons (Z) and the number of neutrons (N), expressed as A = Z + N. Since both protons and neutrons have a mass of approximately 1 atomic mass unit (amu), the mass of an atom in amu is virtually equal to the mass number.
One atomic mass unit is approximately the mass of a proton, or about 1.67 × 10-27 kilograms. Given that electrons have a negligible mass, they do not significantly contribute to the atomic mass. Consequently, when determining the mass of an atom, only the protons and neutrons are typically considered. For instance, a carbon-12 atom, which has 6 protons and 6 neutrons, will have a mass number of 12, and thus, a mass of approximately 12 amu.