Chemical Symbols, Formulae and Reactions

Chemical Symbols

A chemical symbol (sometimes known as an atomic symbol) is a code given to each element, mostly derived from the element’s name. Chemical symbols are typically one or two letters long.

If the chemical symbol of an element only has one letter then that letter is capitalised.

  • For example, the Chemical symbol for the element carbon is C
  • And for Hydrogen is it’s H

However, if the chemical symbol has two letters, then the first letter is uppercase (capitalised) and the second letter is lowercase.

  • For example, the chemical symbol for the element Aluminium is Al
  • And for the element bromine, it’s Br

Keep in mind that some chemical symbols are derived from their Greek or Latin names. For example, the chemical symbol for the element ‘iron’ is Fe. This came from the Latin word ‘ferrum’.

Chemical Formulae

Chemical formulae (the plural of formula) show which and how many atoms join up together to form a compound.

When writing the formula of a molecule, we use the chemical symbols for the elements it contains.

For example with the molecule Aluminium oxide, it is written as AlO, containing one aluminium atom and one oxygen atom.

Using Numbers in Formulae

When a compound contains more than one atom of an element, we use numbers. This shows us how many of atom of an element there is in the compound. These numbers are written smaller than the other letters and numbers in the formula.

For example, with the molecule carbon dioxide. The ‘2‘ in CO2 means that there are two oxygen atoms in the molecule. As there is no number after the carbon atom, this means that there is only one carbon atom.

When using numbers in formulae, it is important to place them at the bottom. For example, with bromine oxide, the formula is written as BrO2, NOT BrO2 or BrO2.

Some formula can be more complicated, meaning they represent molecules which contain multiple elements with more than 1 atom. For example, with sulphuric acid (H2SO4). In this molecule, there are two hydrogen atoms, one sulphur atom, hence the ‘2‘, one sulphur atom, which is why there are no small numbers after sulphur and four oxygen atoms, which explains the ‘4‘.

The formula for a given compound is always the same, that’s what makes a compound unique. For example, if carbon dioxide (CO2) had one oxygen atom instead of three then it would be carbon monoxide (CO) instead.