How is atomic weight defined?

Atomic weight, often referred to as atomic mass, is defined as the sum of protons and neutrons in the nucleus of an atom. This definition considers the two types of nucleons—protons, which are positively charged, and neutrons, which are neutral. The combined number of protons and neutrons gives a substantial measure of the atom's total mass since these particles account for most of the atom's weight, while the mass of electrons is negligible in comparison.

Understanding atomic weight is essential in the field of chemistry and material science, as it plays a crucial role in determining the behavior of atoms during chemical reactions and the formation of compounds. In practice, atomic weight is often expressed in atomic mass units (amu) or unified atomic mass units (u), where one atomic mass unit is defined as one twelfth of the mass of a carbon-12 atom.

The other definitions presented do not accurately capture the concept of atomic weight. For instance, considering only the mass of electrons or solely the number of protons does not provide a complete picture of the atomic constitution. The weight of an element in grams also does not pertain to atomic weight in the context defined here as it applies to the macroscopic scale rather than the atomic scale. Thus