Chemical composition

Chemical composition is the identities and relative amounts of the elements or compounds that make up a substance. It can be expressed as a formula, proportion, or measured concentration.

Chemical element: A pure substance whose atoms all have the same number of protons. Elements are the fundamental identities recorded in a substance’s composition.
Chemical analysis: The determination of the identities or amounts of substances and elements in a sample. It is the broad practice used to establish chemical composition.
Chemical purity: The proportion of a material consisting of a specified substance rather than contaminants or other components. Purity expresses how much of a sample matches its intended composition.
Empirical formula: A chemical formula giving the simplest whole-number ratio of elements in a compound. It reports elemental proportions but can omit a molecule’s actual atom counts.
Chemical compound: A pure substance whose constituent elements are chemically bonded in fixed proportions. Compounds have definite elemental proportions, unlike variable-composition mixtures.
Elemental analysis: A set of methods for measuring the elemental content of a material. It measures the element-by-element makeup directly.
Alloy: A metallic material made from two or more elements, at least one of which is a metal. Alloy properties depend on the identities and proportions of their elements.
Molecular formula: A chemical formula stating the actual number of each kind of atom in a molecule. It gives molecular atom counts rather than only the simplest composition ratio.
Chemical formula: A symbolic notation showing the elements in a substance and, often, their relative numbers. Formulas compactly encode elemental identities and ratios.
Mass spectrometry: An analytical technique that separates ions by their mass-to-charge ratios and measures their abundance. Its signals can reveal molecular identities and constituent proportions.

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