The composition of a compound is given by a chemical formula, which lists the
types of elements contained in each compound, and the relative numbers of each
type of element.
For example, the compound boric acid, which contains the elements boron
(symbol B), oxygen (symbol O), and hydrogen (symbol H) with 1 part boron to 3
parts oxygen and 3 parts hydrogen, has the chemical formula B(OH)3.
Boric acid will always contain the elements boron, oxygen, and hydrogen in this
1:3:3 ratio. Changing either the ratio of these elements or the elements
themselves would produce a different compound.
For example, a more complicated compound, the mineral ulexite, has the
chemical formula
NaCa[B5O6(OH)6].5H2O. This mineral
contains 5 parts boron for each part sodium (symbol Na) and calcium (symbol Ca),
as well as 17 parts oxygen and 16 parts hydrogen.
The properties of compounds are generally different from each other, and from
the properties of the elements from which they have been formed.
