Chemical Formulae
Science: KS3, topic 2.1.23
Chemical Formulae
Instead of writing the element or compound name every time, scientists have symbols for each element, which makes writing about elements easier.
Formulae
- The formulae for compounds are written by putting the element symbols together in a line with no gaps between them.
- Because all compounds are made of elements, this is always the case.
Carbon dioxide
- Carbon dioxide compound is made of carbon and oxygen atoms. The formula is CO2.
- The little numbers tell you how many of each atom is in a compound.
- The number is shown just after the element.
- If there is no number given on the right of an element, then there is just one atom of it.
Water
- Water is made of hydrogen and oxygen atoms. The formula is H2O.
Magnesium oxide
- Any metals usually go first, and any oxygens usually go last.
- The formula of magnesium oxide is:
- MgO.
Magnesium chloride
- MgCl2 is the formula for magnesium chloride, which contains magnesium and chlorine atoms.
- The ‘l’ is a lower case 'L', and not a capital ‘I’. So the compound has magnesium and chlorine, not magnesium, carbon and iodine.
Test yourself on Chemical Formulae
What is the chemical formula for carbon dioxide?
Which of these are true of chemical formulae?
The chemical formula of water is H2O. How many hydrogen and oxygen atoms are in water?
The formula for magnesium chloride is MgCl2. Why is the small number 2 written after the Cl and not after the Mg?