How is atomic mass defined?

Atomic mass is defined as the weighted average mass of an element's isotopes, reflecting both the number of protons and neutrons present in each isotope. It is not simply the number of protons and neutrons, but rather an average that takes into account the different isotopic forms of the element and their relative abundances in nature. This average value is crucial because most elements exist as a mixture of isotopes, which have varying numbers of neutrons, and thus different masses.

For instance, an element like carbon has isotopes such as carbon-12 and carbon-14. The atomic mass of carbon is calculated by weighing the contributions from each of these isotopes based on how frequently they occur. This provides a more precise representation of the mass of an atom as it exists in nature compared to simply counting protons or neutrons individually.

The other options do not accurately define atomic mass: it is not merely the weight in grams, nor is it related to the sum or total of electrons, which are unrelated to the atomic mass fundamentally. Each of those factors — like protons, neutrons, or electrons — plays a role in atomic structure but does not encompass the comprehensive measurement that atomic mass represents.