Chemical Formulae

What's the formula of water? H2O! What does that actually mean and how do we work it out? It means that two hydrogens and one oxygen are bonded together. How do we work that out? The page below answers that question and gets onto some trickier examples. You'll need a periodic table handy.

By the end of this page you should be able to:

  • State that a chemical formulae indicates which atoms are present in a compound and how many of them there are
  • Decode chemical formulae

Why are Co and CO different?

Because new elements begin with a capital letter! So.... Co represents cobalt and CO represents carbon and oxygen joined together. SO represents sulfur and oxygen.

Decoding chemical formulae

Why are Chemists Lazy?

Because they don't write the number one! Going back to H2O from earlier.... It's actually short for H2O1, the number to the right indicates how many there are of that element. So there are two hydrogens and one oxygen. I often ask my students to write in the number one - it often helps! What about H2SO4? H2 means there are two hydrogen, S means that there is one sulfur and O4 means there are 4 oxygen atoms. The diagram to the right contains another example.

Decoding CaCO3


Brackets multiply whatever is inside them. What does this mean? Well if you see (OH)2 in a formula, there is no number next to the oxygen or hydrogen so there is one of each. But the brackets have a 2 to the right so every number inside the brackets is multiplied by two, so there are two hydrogen and two oxygen atoms. What about (SO4)3? There are 3 sulfur atoms and 12 oxygen atoms. Can you see why? If not the diagram to the right might help.

Decoding brackets

There's One Tricky Bit....

Take care with the symbol for chlorine. The periodic table uses a font where capital I and low case l are identical. So CI (carbon and iodine) is identical to Cl (chlorine). How do you know which one it is? Read the question and look for the name of the chemical, simples.

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