How do you write the formula for aluminium chloride?

Hint:. Before writing the chemical formula first we have to write the chemical symbol of elements. The chemical symbol for an element is usually two letters long. The first letter of every chemical symbol is a capital letter and the second letter is a small letter.

Complete step by step answer:
- The chemical symbol is used to represent an atom or an element. The chemical symbol of an element usually consists of one or two letters from the Latin alphabet and the first letter is always capital. There are many sources from which the symbols are derived, it includes Greek name of an element or German name etc. For example: the symbol for lead is Pb and it is derived from the Latin name of lead with is plumbum, the symbol for tungsten is W and it is derived from the German name of tungsten which is Wolfram etc.
- The chemical symbol for aluminium is Al and the chemical symbol for chlorine is Cl. Aluminium chloride is an ionic compound and the ionic compounds are neutral. This means that the sum of charges on each element must be zero.
The charge on aluminium ion = $A{{l}^{3+}}$
The charge on chlorine ion = $C{{l}^{1-}}$
To make the compound neutral, 3 chlorine atoms must be present in the compound. Hence, the symbol of aluminium chloride is $AlC{{l}_{3}}$ .

Note: Sometimes a 3-letter symbol is also assigned to a new element which is a temporary symbol for the element. For example: the chemical symbol for unniloctium was Uno which was later changed to Hs after the discovery of hassium.