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A chemical compound is a chemical substance formed from two or more elements, with a fixed ratio determining the composition. For example, hydrogen oxide (water, Template:Hydrogen2Template:Oxygen) is a compound composed of two hydrogen atoms for every oxygen atom.
A defining characteristic of a compound is that it has a chemical formula. Formulas describe the ratio of atoms in a substance, and the number of atoms in a single molecule of the substance (thus the formula for ethene is Template:Carbon2Template:Hydrogen4 rather than Template:CarbonTemplate:Hydrogen2). The formula does not indicate that a compound is composed of molecules; for example, sodium chloride (table salt, Template:SodiumTemplate:Chlorine) is an ionic compound.
Compounds may have a number of possible phases. All compounds can exist as solids. Molecular compounds may also exist as liquids or gases. All compounds will decompose to smaller compounds or individual atoms if heated to a certain temperature (called the decomposition temperature). Every chemical compound that has been described in the literature carries a unique numerical identifier, its CAS number.