Compounds, formulae and equations (2.1.2)

To determine the formula for an ionic compound the following guidelines should be observed:

• There should be no overall charge (the number of positive and negative charges should cancel to create a neutral uncharged ionic compound).
• The positive ion (cation) is always placed first (on the left-hand side) and the negative ion (anion) is always on the right-hand side.
• Brackets are used to keep the component parts of a polyatomic ion together, for example
  or
• Subscript numbers multiply the ion on the left of the bracketed contents.

The following non-metallic elements exist as diatomic molecules (two atoms covalently bonded together):

When referred to by the element name such as nitrogen this refers to the diatomic molecule therefore when drawing the formula should be shown as .

Oxygen, , can also be observed as ozone, .

Some elements exist as multiple atoms covalently bonded in a single molecule, known as polyatomic molecules. The ones you are expected to recall are:

• phosphorus:
• sulfur: .

The formulae for the reactant(s) are written on the left-hand side of an equation, and the right-hand side of the equation features the formulae for the product(s) of the reaction.

State symbols show the physical state of reactants and products in a chemical reaction and should be used as required.

Conservation of mass means that no atoms are created or destroyed in a chemical reaction; the total number of atoms should be the same on both sides.

The two halves of an equation – the reactants and products – are separated by an arrow.

This shows the direction of the reaction.

Alternatively, the reversible reaction symbol is used, if the reaction does not go to completion.

Balancing equations is a process to make sure there are the same number of atoms on each side of the symbol equation therefore obeying the law of conservation of mass.

When balancing equations ensure:

• the only numbers changes are the coefficients. These are placed in front of the formulae
• the formulae are not altered in any way, such as changing to to increase the number of oxygen atoms.

To convert a balanced (stoichiometric) symbol equation into an ionic equation we must:

• split compounds composed of aqueous ions into the individual ions, including any multiples
• leave intact in the equation any substance that does not contain aqueous ions or is covalently bonded
• cancel out spectator ions which appear on both sides of the equation and do not undergo any change.

When balancing equations, we must:

• identify the number of atoms of each element in the reactants and products
• add the appropriate coefficient in front of the formulae to balance the number of atoms on each side
• make sure that the final balanced equation has the same number of each type of atom on both sides.