Chemical substances are represented by chemical formulae. The formula
indicates which elements form the substance and in what proportion these
The formula of the water, H2O, shows that it is formed by hydrogen and
oxygen, and that there is an atom of oxygen for every two atoms of hydrogen.
Atoms can get the configuration of a noble gas in three ways: obtaining,
lending or sharing electrons with other atoms.
In the elements of the representative groups (alkali metals, alkaline earth
metals, group of B, group of C, group of N, chalcogens and halogens) the outer
level is completed with eight electrons. The atoms with few outer electrons
(alkali metals, alkaline earth metals) lend them giving place to positive ions (cations)
and they form ionic compounds. Atoms with many outer electrons (halogens,
chalcogens, etc.) obtain electrons giving place to negative ions (anions), they
form ionic compounds with metals, but covalent compounds with nonmetals.
Atoms with an intermediate number of outer electrons (for example carbon)
share electrons with other atoms becoming compounds covalent.
The compounds are electrically neutral, except ions. The charge that all the
atoms of a compound contribute has to be globally neutral, there are as many
positive charges as negative ones.
To know which is the charge that every atom contributes, we use the oxidation
The oxidation number is a number that indicates the number of electrons that
an atom uses when it forms a compound.
The oxidation number is positive if the atom lends electrons, or shares with
a more electronegative atom. And it will be negative when the atom gains
electrons, or shares them with an atom less electronegative.
The oxidation number is written in Roman numbers (remember it in the
nomenclature of Stock): +I, +II, +III, +IV,-I,-II,-III,-IV, etc. But in this web
also we use Arabic characters for them: +1, +2, +3, +4, -1, -2, -3, -4. etc., as
if they were whole numbers.
In monoatomic ions the electrical charge coincides with the oxidation number.
In the oxidation number we write the sign + or - it on the left of the number,
like in the whole numbers. The charge of the ions, or charge number, must be
written with the sign on the right of the digit: Ca2+ ion
calcium(2+), CO32- ion carbonate(2-).
How can we know the oxidation number that corresponds to each atom? It is
enough to know the oxidation number of the elements that have fixed oxidation
number, which are a few, and it is very easy to deduce it from the electronic
configurations. These oxidation numbers appear in the following table. The
oxidation numbers of the other elements are deduced from the formulae or are
indicated in the name of the compound.