Formulae and Equations

It is always useful to know what elements make up a compound and just by learning a few simple rules it is a lot easier to name and recognise them:

• A compound made up of only two elements will end in –IDE, e.g. sodium chloride.
• When naming a compound, the metal is always written first.
• If oxygen is present the name will end in –ATE (or sometimes –ITE), e.g. magnesium sulphate contains magnesium, sulphur and oxygen.

Below are some examples of compounds with the elements stated that they contain:

• copper oxide – copper and oxygen
• hydrogen sulphide – hydrogen and sulphur
• sodium phosphate – sodium phosphorus and oxygen
• caesium chloride – caesium and chlorine
• sodium hydrogencarbonate – sodium, hydrogen, carbon and oxygen

 It can also be done the other way round:

• sodium and oxygen – sodium oxide
• zinc and sulphur – zinc sulphide
• oxygen and potassium – potassium oxide
•  nickel, phosphorous – nickel phosphate
• chlorine and copper – copper chloride

Ø  Do you know your ions?

Remember! An ion is a charged particle that has either lost or gained electrons.

The following is a table of some common ions that you might come across. Most of them you can work out from their position in the periodic table, but others you will just have to learn (who said Chemistry was fun?!)

Ø  The Formula!

When working out the formula for an ionic compound, follow these three simple steps:

• Look at the name of the compound
• Decide which ions are in it
• Balance the charges

Look at the following examples:

Ø  Compounds containing complex ions

Ions such as carbonate, CO₃^2- and sulphate, SO₄^2- are known as complex ions as they contain more than one atom.

When balancing a formula that contains a complex ion, you need to put it in brackets, e.g. if you need two nitrates to balance a 2+ charge then you write it as (NO₃)_{2 ….}not NO₃₂! Below are a couple more examples:

 

Ø  Formulas of molecules

Molecules are made up of atoms not ions so when working out the formula we can’t use the balancing the charges method as shown before. Instead we have to use its position in the periodic table to work out how many bonds it will form:

• Carbon is in group 4 of the periodic table; it has 4 electrons in its outer shell so it can form 4 bonds to get a complete outer shell.
• Chlorine is in group 7 of the periodic table; it has 7 electrons in its outer shell so it forms 1 bond to get a full outer shell.
• So 1 carbon atom can bond with 4 chlorine atoms so the formula is CCl₄.

Here are some more examples:

• Hydrogen and bromine - hydrogen is in group 1 of the PT, it has 1 electron in its outer shell so forms 1 bond to get a full outer shell. Bromine is in group 7 of the PT, it has 7 electrons in its outer shell so forms 1 bond to get a full outer shell. So 1 hydrogen atom can bond with 1 bromine atom so the formula is HBr.
• Nitrogen and hydrogen – nitrogen is in group 5 of the PT, it has 5 electrons in its outer shell so forms 3 bonds to get a full outer shell. As said above, hydrogen can form 1 bond to get a full outer shell. So 3 hydrogen atoms can bond with 1 nitrogen atom so the formula is NH₃.
• Phosphorous and oxygen – phosphorous is in group 5 of the PT, it has 5 outer shell electrons so can form 3 bonds. Oxygen is in group 6, it has 6 outer shell electrons so can form 2 bonds. 2 phosphorous atoms can bond with 3 oxygen atoms so the formula is P₂O₃.

Ø  Just one last bit…. balancing equations

A chemical equation tells us the reactants and products for that reaction.

When they are balanced they can also tell us the amount of each reactant and product.

Follow these few steps and you won’t go far wrong (fingers crossed):

• Write down the word equation for the reaction.
•  Now write down the (correct) formulas for each compound in the equation.
• Now make sure that the number of atoms of each element are the same on both sides i.e. balanced.