When atoms of the same type come together they form elements, but when different atoms come together they form compounds….and that just added a whole new level of complication to your Chemistry revision!

Atoms Join Together To Make Compounds

When two different elements react their atoms form chemical bonds – the new substance we call compounds. It’s usually pretty damn difficult to separate compounds back out into their constituent parts.

When a chemical bond is formed then an electron is taken, given away or shared. The nuclei never get involved.

Ionic and Covalent Bonding

When a metal and a non-metal are involved then a compound consisting of ions is formed. The metal atoms give away an electron and form positive ions. The non-metal atoms gain electrons to form negative ions. The opposite charges of these ions means that they are attracted to each other…it’s called IONIC BONDING.

Here’s an example of how sodium and chlorine bond to become sodium chloride by sodium giving away an electron…which you might know better as simply ‘salt’!


sodium chloride ionic bonding

A compound formed of just non-metals consists of molecules. Here each atom shares an electron with another atom.’s called COVALENT BONDING. The atoms make enough of these bonds to fill up their outer shells.

Let’s look at water as an example. It’s made up of hydrogen and oxygen. If you look at the periodic table you’ll see oxygen has 8 protons and therefore 8 electrons (see here if need a refresher on this and electron shells). So its electrons shells are 2,6. This means it needs two more electrons to complete it’s outer shell. Hydrogen only has one electron, so it needs just one electron to complete its outer shell. Therefore an oxygen atom will combine with two hydrogen atoms so they all have a complete outer electron shell.

water molecule


So you get two hydrogen to every oxygen atom. Hence H2O.

It’s pretty weird, but the properties of compounds are often totally different to those of their original elements. Just think about how different water is from chlorine and oxygen!

Compound Formulas

Compounds are often written as formulas and these tell you how the molecules of a particular compound are made up:

  • CO2 – this is carbon dioxide and made up of one carbon and two oxygen atoms.
  • H2SO4 – this is sulfuric acid and you can see it is made up of two hydrogen, one sulphur and four oxygen atoms.
  • Ca(OH)2 – This is calcium hydroxide. Don’t be put off by the brackets, it just means the number outside the brackets applies to everything inside the brackets. So calcium hyrdoxide is made up on one calcium, two oxygen and two hydrogen atoms.