Atommass

Atommass refers to the mass of a single atom. This mass is primarily determined by the number of protons and neutrons within the atom's nucleus, as electrons contribute negligibly to the overall mass. The standard unit used to express atomic mass is the atomic mass unit (u), also known as a Dalton (Da). One atomic mass unit is defined as one-twelfth the mass of an unbound neutral atom of carbon-12 in its ground state.

Isotopes of an element have the same number of protons but different numbers of neutrons, leading to different atomic masses. Therefore, a sample of an element found in nature typically consists of a mixture of isotopes. The atomic mass listed on the periodic table for an element is not the mass of a single atom but rather the weighted average of the masses of its naturally occurring isotopes, taking into account their relative abundances. This average atomic mass is crucial for stoichiometric calculations in chemistry. For example, the atomic mass of oxygen is approximately 15.999 u, reflecting the combined masses and abundances of its isotopes like oxygen-16, oxygen-17, and oxygen-18.