The name of a compound is useful as it usually tells us some of the elements that the compound contains.
Coefficient and subscript
Coefficient is the number in front of the formula. It indicates the number of molecules or ions of each kind involved in the reaction.
Subscript is the number to the lower right of the chemical symbol. It indicates the number of atoms of that element found in the compound.
Every compound has a chemical formula (when we talk about more than one formula, we refer to them as formulae).
Chemical formulae are put together using the chemical symbols for the elements that make up the compound.
A chemical formula shows the ratio of atoms of each element in the compound. For example: zz
The chemical formula for
water is H2O. This formula shows that for every oxygen atom there are two hydrogen atoms.
z z The chemical formula for
carbon dioxide is CO2. The formula shows that for every carbon atom there are two oxygen atoms.
z
Fig. 14.4.1 The 2 in H2O is a subscript
When writing a chemical formula, the numbers of atoms in the compound should be on the right-hand side of the chemical symbol and written as subscript (in a smaller size). If we put ‘2’ in front of a formula it means we have two molecules. For example, 2H20 = (H20 + H20).
Chemical formulae
Copy and complete the table by giving the name of the elements that make up the following substances and the ratio for each element in the compound: sodium chloride (NaCl), silver chloride (AgCl), carbon dioxide (CO2), magnesium oxide (MgO), lead iodide (PbI2), aluminium chloride (AlCl3) Example:
Name of compound Chemical formula Elements present Ratio of elements present Sodium sulfide
Na2S
Sodium Sulfur
Na : S 2 : 1
Predict the ratio of elements in a compound using the periodic table. Produce chemical formulae for compounds of two elements.
