Gidemy

Chemical Formulae

The chemical formula of a compound shows how many of each type of atom join together to make the units which make up the compound.

• For example, in iron sulfide every iron atom is joined to one sulfur atom, so we show its formula as FeS. In sodium oxide, there are two sodium atoms for every oxygen atom, so we show its formula as Na₂O. Notice that the ‘2’ is written as a subscript, so Na2O would be wrong.

• This diagram shows that one carbon atom and two oxygen atoms combine to make up the units of carbon dioxide. Its chemical formula is written as CO₂.

• Carbon dioxide units contain one carbon atom and two oxygen atoms

• Sometimes you see more complex formulae such as Na₂SO₄ and Fe(OH)₃:a unit of Na₂SO₄ contains two sodium atoms, one sulfur atom and four oxygen atoms joined together a unit of Fe(OH)₃ contains one iron atom, three oxygen atoms and three hydrogen atoms – the brackets show that the 3 applies to O and H

Writing a chemical Formulae

• The chemical formula of a covalent molecular substance gives the number of atoms per molecule. The formula of a covalent network or ionic compound gives the simplest ratio of atoms/ions in the substance.
• Sometimes the name of the compound gives information about the formula of that compound. Names of these compounds have prefixes that give the number of atoms of certain elements in each molecule.

Prefix	Number of atoms
Mono-	One
Di-	Two
Tri-	Three
Tetra-	Four
Penta-	Five
Hexa-	Six

• For example, carbon monoxide contains one carbon atom joined to one oxygen atom, so it has the formula CO.

Valency and Chemical Formulae

Valency is the combining power of an element. Elements in the same group of the periodic table have the same valency. The valency of an element is related to how many electrons are in the outer shell.

Group number	1	2	3	4	5	6	7	8
Valency	1	2	3	4	3	2	1	0

• The noble gases have the valency 0 as they do not usually combine with other elements.
• To write the chemical formula for a compound it is best to use the S.V.S.D.F system.

Step one – write down the symbols of both the elements involved.

Step two – beneath each symbol, write it’s valency. Memorising the above table is useful.

Step three – swap the valencies over.

Step four – if the valencies can be simplified, divide them both by the smaller of the two numbers. If one of the numbers is already one, then they cannot be divided and simplified any further.

Step five – write the formula.

Example one

• What is the formula for potassium oxide?

Example two

• What is the formula for aluminium oxide?

Example three

• What is the formula for carbon sulfide?

Formulae of compounds containing group ions

• Group ions contain two or more atoms and usually have a negative charge. The formulae of these ions can be found in the data booklet. We can take the number of charges on the ion as the valency of the ion.

Example

What is the formula for calcium nitrate?

Formula using Roman numerals

• Some elements, particularly the transition metals, do not always have the same valency in their different compounds. The valency of these elements is usually given in roman numerals inside brackets.

Roman Numeral	Number
I	One
II	Two
III	Three
IV	Four
V	Five
VI	Six

Example

What is the formula for copper (I) oxide?