The terms "atomic mass" and "atomic weight" are often used interchangeably, but they have slightly different meanings:
Atomic Mass: Atomic mass refers to the mass of a single atom of an element. It is typically expressed in atomic mass units (u) or unified atomic mass units (u), where one atomic mass unit is defined as 1/12th the mass of a carbon-12 atom. The atomic mass of an atom takes into account the masses of its protons, neutrons, and electrons.
Atomic Weight: Atomic weight is the weighted average of the masses of all the naturally occurring isotopes of an element. Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei. Atomic weight considers the abundance of each isotope in nature to calculate a weighted average. Atomic weight is also expressed in atomic mass units (u) or unified atomic mass units (u).
The key difference between atomic mass and atomic weight is that atomic mass refers to the mass of a single atom, while atomic weight is the average mass of all the isotopes of an element, taking into account their respective abundances.
It's worth noting that atomic weight is sometimes referred to as relative atomic mass or standard atomic weight, depending on the context.