atómmassi

Atómmassi, also known as atomic mass, is the mass of an atom. It is usually expressed in atomic mass units (u, also called daltons) or, for larger quantities, in kilograms. By definition, 1 u equals one twelfth of the mass of a carbon-12 atom, about 1.6605 × 10^-27 kg. The atomic mass of a specific isotope is close to the sum of the masses of its protons and neutrons, with electrons contributing only a small amount. Because of binding energy, the nucleus is slightly lighter than the sum of its nucleons, a difference known as the mass defect.

Elements occur as isotopes with differing numbers of neutrons. The atomic mass commonly listed for an element on the periodic table is a weighted average of the masses of its naturally occurring isotopes, accounting for their abundances. This quantity is called the standard atomic weight or relative atomic mass, and is typically given in u or, equivalently, in g/mol (the molar mass). For example, carbon has an average atomic mass of about 12.01 u and a molar mass of roughly 12.01 g/mol.

Atomic mass is determined in part by mass spectrometry, which measures isotopic composition and mass-to-charge ratios of ions. In chemical calculations, the molar mass in g/mol is numerically equal to the relative atomic or molar mass in u. Understanding atómmassi supports stoichiometry, molecular weight calculations, and the interpretation of physical properties of elements and compounds.