Atomfraktion

Atomfraktion, or atomic fraction, is a dimensionless quantity that expresses the proportion of atoms of a given element or component in a system relative to the total number of atoms. It is defined as x_A = N_A / ∑_i N_i, where N_A is the number of atoms of element A and the sum runs over all atoms in the system.

Atomic fraction can be distinguished from mole fraction. It counts atoms, which may differ from the moles of chemical species in systems where molecules contain multiple atoms. In a compound, atomic fractions reflect the ratio of atoms in the formula unit; for example, in Fe3O4 there are 3 iron atoms and 4 oxygen atoms, so x_Fe = 3/7 ≈ 0.4286 and x_O = 4/7 ≈ 0.5714.

Applications of atomic fractions occur in chemistry, materials science and related fields. They are used to describe alloy compositions, semiconductor doping, and the isotopic makeup of a sample, as well as to perform stoichiometry, diffusion modeling and surface analyses. Atomic fractions are often obtained from experimental techniques that quantify elemental composition, such as electron probe microanalysis (EPMA), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS), or inferred from known chemical formulas.

Relation to other fractions: atomic fractions can be converted to mass fractions via w_A = x_A M_A / ∑_i x_i M_i, where M_A is the atomic mass of element A. In isotopic contexts, the term may refer to isotopic abundance—the fraction of atoms that are a given isotope—though this is often referred to as isotopic fraction or abundance.

Limitations include non-stoichiometric materials or partial occupancies, where x_A reflects the average occupancy rather than a fixed count of atoms.