What is atomic weight?

Atomic weight, also referred to as atomic mass, is defined as the average mass of an atom's isotopes, which takes into account both the mass of protons and neutrons within an atom's nucleus. This average incorporates the relative abundances of each isotope of an element, making it a weighted average rather than a simple sum.

The reason option B is correct is that the atomic weight explicitly reflects the composition of an atom in terms of its fundamental particles. Protons and neutrons each contribute approximately one atomic mass unit (amu) to the total weight of the atom. Electrons have a negligibly small mass compared to protons and neutrons and are often not included in the calculation of atomic weight.

The other choices represent different concepts: the total number of atoms in a molecule does not define atomic weight, as molecular counts relate to chemical formulas rather than individual atomic masses. The sum of the atomic masses of all the elements in a compound relates more to the molecular weight of a compound rather than the weight of an individual element. Lastly, while the average mass of an atom's isotopes does describe a core concept related to atomic weight, the most accurate and accepted definition specifically incorporates the idea of isotopic abundance; hence, it aligns closely