How is atomic weight determined?

Atomic weight, also referred to as atomic mass, is determined by the combined total of protons and neutrons within an atom's nucleus. This is because both protons and neutrons contribute significantly to the mass of an atom, with protons having a mass of approximately 1 atomic mass unit (amu) and neutrons having a similar mass.

Electrons, while present in an atom and contributing to its overall charge, have a negligible mass compared to protons and neutrons and therefore do not significantly affect the atomic weight.

Additionally, the number of protons defines the element and its position on the periodic table, while neutrons contribute to the atomic mass but do not change the chemical identity of the element unless their number changes, in which case the atom becomes an isotope. This understanding of how atomic weight is calculated is fundamental in chemistry and is crucial for applications such as stoichiometry and thermodynamics.