Chemical formulae

Chemical substances are represented by chemical formulae. The formula indicates which elements form the substance and in what proportion these elements combine.
 

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.

Aims of the formulation

The aim of formulation and chemical nomenclature is that the name of a compound indicates us which is its formula, and from this one we know its name. The rules of the I.U.P.A.C. (International Union of Pure and Applied Chemistry) make formulation easier.

Why do the atoms bond?

To get more stability (to be more stable).

The electronic configurations of the atoms inform about the electrons of the last layer, which are important for being those which take part in the chemical bonds and in the chemical reactions. The noble gases are very stable for having its electronic layers complete . Having complete electronic layers will make all the atoms more stable when forming compounds.

How do the atoms get the configuration of a noble gas?

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.

How many atoms will be combined in a compound?

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 number.

    What is the oxidation number? 

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+), CO₃^2- 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.

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