When numbers are written as 'subscripts' in compounds (i.e. they are written below and to the right of the element symbol), this tells us how many atoms of that element there are in relation to other elements in the compound. For example in nitrogen dioxide (
) there are two oxygen atoms for every one atom of nitrogen. In sulphur trioxide (
), there are three oxygen atoms for every one atom of sulphur in the compound. Later, when we start looking at chemical equations, you will notice that sometimes there are numbers
before the compound name. For example,
means that there are two molecules of water, and that in each molecule there are two hydrogen atoms for every one oxygen atom.
We can use these rules to help us name both ionic compounds and covalent compounds (more on these compounds will be covered in a later chapter). However, covalent compounds are often given other names by scientists to simplify the name (or because the molecule was named long before its formula was discovered). For example, if we have 2 hydrogen atoms and one oxygen atom the above naming rules would tell us that the substance is dihydrogen monoxide. But this compound is better known as water! Or if we had 1 carbon atom and 4 hydrogen atoms then the name would be carbon tetrahydride, but scientists call this compound methane.
What is the chemical name for
For a) we have potassium and the permanganate ion. For b) we have the ammonium ion and chlorine.
For a) we list the potassium first and the permanganate ion second. So a) is potassium permanganate. For b) we list the ammonium ion first and change the ending of chlorine to -ide. So b) is ammonium chloride.
Write the chemical formulae for:
sodium sulphate
potassium chromate
In part a) we have
(sodium) and
(sulphate). In part b) we have
(potassium) and
(chromate)
In part a) the charge on sodium is
and the charge on sulphate is
, so we must have two sodiums for every sulphate. In part b) the charge on potassium is
and the charge on chromate is
, so we must have two potassiums for every chromate.
a) is
and b) is
Naming compounds
The formula for calcium carbonate is
.
Is calcium carbonate a mixture or a compound? Give a reason for your answer.
What is the ratio of
atoms in the formula?
Give the name of each of the following substances.
Give the chemical formula for each of the following compounds.
potassium nitrate
sodium iodide
barium sulphate
nitrogen dioxide
sodium monosulphate
Refer to the diagram below, showing sodium chloride and water, and then answer the questions that follow.
What is the chemical formula for water?
What is the chemical formula for sodium chloride?
Label the water and sodium chloride in the diagram.
Give a description of the picture. Focus on whether there are elements or compounds and if it is a mixture or not.
