To write the correct formula for something, you need to know
a) how is it bonded?
b) what are the charges on each component?
c) what is the chemical symbol for each component?
Here's how we do it:-
Here's a compound ammonium hydroxide
Now, how is it bonded? Ionic
The way we write ammonium is NH4 and it has a single positive charge.
Similarly, hydroxide is written OH and has a single negative charge.
All we need is for there to be equal positive and negative charges in the compund. There is one on ammonium and one on hydroxide, so we are balanced. Therefore, the formula is NH4OH.
At GCSE, it is best to know these elements off by heart:-
Silver (Ag+), Ammonium (NH4+), Zinc (Zn2+), Hydrogen (H+), Hydroxide (OH-), Nitrate (NH3-), Sulphate (SO42-) and Carbonate (CO32-)
It's all very well having formulae, but it's important to make sure you end up with the right amount and balance the equation, like scales.
Take a look on this equation:-
FeS + HCl --> FeCl2 + H2S
You may notice that we actually have more hydrogen and chloride on one side than the other, so we need to balance it up. On one side we have one hydrogen but on the other we have two, so we must have started with two hydrogens. There's also two chlorides which means we must have started with two chlorides. Since hydrogen and chloride are within the same compund (hydrogen chloride) to begin with, we can just times it by two, to achieve a balanced equation.
FeS + 2HCl --> FeCl2 + H2S
This is a balanced equation!
A type of bonding that you could use formulae and equation skills is covalent bonding.
The bonding of two non-metals.Take Ammonia for example:-
We have a nitrogen atom with 5 electrons and a hydrogen atom . Both would like full outer energy levels, so have to share electrons.
Nitrogen needs 3 electrons and hydrogen needs 1. Hydrogen only has one electron so nitrogen will have to share with 3 hydrogen atoms. At the same time, nitrogen will share an electron with hydrogen giving it a full outer energy level like this:-
Both have achieved full outer energy levels to create ammonia: NH3
Diamond and Graphite
In an exam you may be asked to quote some of these features of diamond and graphite:
Diamond: Diamond is a form of carbon used for cutting tools and jewellery. It is made up of covalent bonds which are strong. All four bonds are used in the carbon atom, creating a giant covalent 3D structure. As a result, diamond has a high melting point because of the strong covalent bonds, as well as being hard and abrasive. It cannot conduct electricity because it has no delocalised electrons to carry the charge.
Graphite: Another form of carbon used however as a lubricant and electrodes for electrolysis. It is made up covalently and three of the four bonds are used created a layered structure. This means that although it has a high melting point because of the strong bonds, it can conduct electricity because each carbon atom only has 3 out of the 4 bonds made, leaving an electron delocalised and free to move and carry charge. The forces of attraction between layers is weak and layers can easily be broken off.