How to write Chemical Formula
Core
• Use the symbols of the elements and write the formulae of simple compounds.
Elements are classified into a chart called the Periodic Table. Every element is given a name and a chemical symbol.
First letter in chemical symbol is always capitalised; any other letter that follows is in lower case
For two-letter symbols, they are not always the first two letters of the name of the elements,
E.g. symbol for Chlorine is Cl, not Ch.
Formulae of element
Metals exist as atoms. E.g. formula for sodium is Na, copper is Cu and iron is Fe.
Group O elements also exist as atoms. E.g. formula for Helium is He and Argon is Ar.
Some non- metals such as hydrogen, nitrogen, oxygen and all Group VII elements exit as diatomic molecules. E.g. formula for hydrogen is H2, oxygen is O2 and chlorine is Cl2.
Formula of some common covalent compounds
You should know how to write simple formula of covalent substances from the chapter of covalent bonding.
Some examples:
| Covalent compounds |
Formula |
| Water |
H2O |
| Methane |
CH4 |
| Ammonia |
NH3 |
| Hydrogen chloride |
HCl |
For formula of covalent compounds with oxygen, the name will usually suggest how many oxygen atoms inside the molecule. ‘Mono’ represents one oxygen atom, ‘di’ means two oxygen atoms and ‘tri’ mean three oxygen atoms.
Some examples:
| Name of compound |
Formula |
| Carbon monoxide |
CO |
| Nitrogen monoxide |
NO |
| Carbon dioxide |
CO2 |
| Sulfur dioxide |
SO2 |
| Nitrogen dioxide |
NO2 |
| Sulfur trioxide |
SO3 |
Formulae of some ionic compounds
Make the number of positive a of the cations and negative charges of anions the same.
Eg 1: Sodium fluoride is made of Na+ and F- ions. The number of positive and negative charges are the same, so the formula is NaF.
Eg 2: Calcium oxide is made of Ca2+ and O2- ions. The number of positive and negative charges are the same so the formula is CaO.
Eg 3: Aluminium sulfide is made of Al3+ and S2- ions. To make the number of charges the same, we need two Al3+ ions and three S2- ions so that there will be 6 positive and 6 negative charges, so the formula is Al2S3.
Formulae of some ionic compounds involving polyatomic ions
Some common polyatomic ions
| Ammonium |
NH4+ |
| hydroxide |
OH- |
| nitrate |
NO3- |
| carbonate |
CO32- |
| sulfate |
SO42- |
If more than one polyatomic ion is needed, the formula involving this polyatomic ion is written inside brackets with the number of the ion needed.
Eg 1: Magnesium hydroxide is made of Mg2+ and OH- ions. To make the number of charges the same, we need one Mg2+ ion and two OH- ions, the formula is Mg(OH)2.
Eg 2: Calcium sulfate is made of Ca2+ and SO42-ions. The number of positive and negative charges are the same, so the formula is CaSO4. There is no need for any bracket.
Eg 3: Aluminium nitrate is made of Al3+ and NO3- ions. To make the number of charges the same, we need one Al3+ ions and three NO3- ions. There will be 3 positive and 3 negative charges, the formula is Al(NO3)3.
Eg 4: Ammonium carbonate is made of NH4+ and CO32- ions. To make the number of charges the same, we need two NH4+ ions and one CO32-ions. There will be 2 positive and 2 negative charges, the formula is (NH4)2CO3.