Writing Simple Chemical Formulae 1


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The names of chemicals have different endings. The ending of the chemical name gives an indication of the number of elements present in the compound.

In this tutorial we will deal with chemical compounds that contains only two elements and their names all end in the letters -IDE. (an exception to this are hydroxides such as sodium hydroxide which actually contains more than  two element. This will be dealt with in another tutorial.)

A two-element compound made from sodium and chlorine will be called sodium chloride as sodium is a metal and metals always appear first in the name. If both elements in the compound are non-metal elements, it is less important which element is named first e.g. water could be called hydrogen oxide or oxygen hydride.

The first step is to identify the elements in the compound.

Compound Element 1 Element 2
potassium bromide potassium bromine
calcium chloride calcium chlorine
carbon oxide carbon oxygen
aluminium nitride aluminium nitrogen

Then we need to know a term called valency number which refers to the combining power of the element. There are two rules to help us based on the position of the element in the periodic table.

Elements in Groups 1, 2, 3 and 4 have a valency number (VN) of 1, 2, 3 and 4 respectively.

Elements in Groups 5, 6 and 7 have a VN of 3, 2 and 1 respectively.

Example 1

Potassium in Group 1 and has a VN = 1. Bromine is in Group 7 and has a VN of 1.

To write the formula put the symbols under the name of the element and then put in the valency numbers. Finally, cross over the numbers and write the formula by putting the numbers after the symbol and in 'subscript' format. Any 1's that appear in the formula can be missed out

Potassium         bromine 

   K                      Br

   1                        1


So the formula is KBr.

Example 2

Calcium chloride contains calcium (Group 2, VN =2 ) and chlorine (Group 7, VN = 1)

Calcium              chlorine

   2                          1

               Ca1Cl2 which becomes CaCl2

After a while it becomes common practice not to bother showing '1' in a formula.

Example 3

Carbon oxide contains carbon (Group 4, VN = 4) and oxygen (Group 6, VN = 2)

Carbon                oxygen

    4                          2

          C2O4       This formula can be simplified as the ratio of C:O of 2:4 is simplified to a ratio of 1:2


Example 4

Aluminium nitride contains Aluminium (Group 3, VN = 3) and nitrogen (Group 4, VN = 3)

Aluminium            nitrogen

    3                            3

                Al3N3 which becomes AlN