Chemists describe compounds using formula in various formats. For molecules, the formula for the molecular unit is shown. For polymeric materials, such as minerals and many metal oxides, the empirical formula is given, e.g. NaCl for table salt. The order of the elements in molecular and empirical formulas is C, then H and then alphabetical. Trifluoroacetic acid is thus described as C2HF3O2. More descriptive formulas convey structure information, illustrated again with trifluoroacetic acid. CF3CO2H. On the other hand, formulas for inorganic compounds often do not convey structural information, as illustrated by H2SO4 for a molecule that has no H-S bonds. A more descriptive presentation would be O2S(OH)2. Elements form compounds to become more stable. They become stable when they have the maximum number of possible electrons in their outermost energy level, which is normally two or eight valence electrons. This is the reason that noble gases do not frequently react: they already possess eight valence electrons (the exception being helium, which requires only two valence electrons to achieve stability).
Compounds may have several possible phases. All compounds can exist as solids, at least at low enough temperatures. Molecular compounds may also exist as liquids, gases, and, in some cases, even plasmas. All compounds decompose upon applying heat. The temperature at which such fragmentation occurs is often called the decomposition temperature. Decomposition temperatures are not sharp and depend on the rate of heating. At sufficiently high temperatures, all compounds, either after they have decomposed somehow or in the act of decomposing, fragment into smaller compounds or to individual atoms.
Every chemical compound carries a unique numerical identifier, known as its CAS number.