Compound
A pure substance formed when two or more different elements combine chemically in a fixed, whole-number mass ratio.
Unit
Dimensionless
In depth
A compound's properties bear no resemblance to those of its constituent elements, because chemical bonding rearranges the valence electrons entirely. Compounds can only be separated into their elements by chemical or electrolytic means, never by physical methods such as filtration or distillation.
Examples in the real world
Sodium chloride from the metal sodium and the toxic gas chlorine; water from two explosive and combustion-supporting gases.