In general, this fixed ratio must be fixed due to some sort of physical property, rather than an arbitrary man-made selection. This is why materials such as brass, the superconductor YBCO, the semiconductor Aluminium gallium arsenide or chocolate are considered mixtures or alloys rather than compounds.
A defining characteristic of a compound is that it has a chemical formula. Formulas describe the ratio of numbers of atoms in a substance. For example, in H2O (water) there are two hydrogen atoms for every one oxygen atom. The formula does not tell you that water is made of molecules. Indeed water ice has the same formula, but it is in the form of a crystal - there are no molecules in ice.
Compounds may have a number of possible phases. For a compound to be a liquid or a gas and still be called a compound, atoms from the various elements must be stuck together in the form of molecules. The formation of molecules is why compounds such as C2H4 exist (rather than just CH2) - the formula is telling you not just the ratios but also how many atoms there are in each molecule.
Every chemical compound that has been described in the literature carries a unique numerical identifier, its CAS number.
Types of compounds:list of compounds for a list of all compounds currently in Wikipedia.\n