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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 atoms in a substance, and the number of atoms in a single molecule of the substance (thus the formula for ethene is C _} H _} rather than C H _}). The formula does not indicate that a compound is composed of moleculeIn science, a molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. A molecule consists of multiple atoms joined by shared pairs of electrons in a covalent bond''. It may consist of atoms; for example, sodium chlorideProperties General Name Sodium chloride Chemical formula Na Cl Appearance White or clear solid CAS-number 7647-14-5 Physical Formula weight 58. 4 amu Melting point 1074 K (801 °C) Boiling point 1738 K (1465 °C) Density 2. 2 ×103 kg/ m3 Crystal structure f (table salt, Na Cl) is an ionic compoundIn chemistry, an ionic compound is a chemical compound in which ions are held together in a lattice structure by ionic bonds. Ions can be single atoms, as in common table salt sodium chloride, or more complex groups such as calcium carbonate. But to be co.

Compounds may have a number of possible phases. Most compounds can exist as solidA solid is a state of matter, characterized by a definite volume and a definite shape (i. it resists deformation). Within a solid, atoms/ molecules are relatively close together, or "rigid"; however, this does not prevent the solid from becoming deformeds. Molecular compounds may also exist as liquidOne of the four phases of matter, a liquid is a fluid whose volume is fixed under conditions of constant temperature and pressure; and, whose shape is usually determined by the container it fills. Furthermore, liquids exert pressure on the sides of a conts or gasFor other meanings see gas (disambiguation). A gas is one of the phases of matter. Gases are, like liquids, fluids: they have the ability to flow and do not resist deformation. Unlike liquids, however, unconstrained gases do not occupy a fixed volume, butes.

All compounds will decompose to smaller compounds or individual atomFor alternative meanings see atom (disambiguation). An atom is a microscopic structure found in all ordinary matter around us. Atoms are composed of 3 types of subatomic particles: electrons, which have a negative charge; protons, which have a positive chs 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.