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,H2O} 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 C2H4 rather than CH2). The formula does not indicate that a compound is composed of molecules; for example, sodium chloride (table salt, NaCl is an ionic compound.
Compounds may have a number of possible phases of matter[]. 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[].
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