Chemical Formula of compounds and rules for writing

  Chemical Formula:

The symbolic representation of composition is termed as the chemical formula of a compound.

Two things are required to learn and write chemical formula:

1.  Ability to write symbols, which one can attain after good practice over learning and memorising symbols.

2.  Combining capacity of the element of compound or valency.

Valency :

The combining power or capacity of an element is termed as its valency.

It helps in finding out, how the atoms of an element will combine with atom(s) of the another element to form a reaction or any chemical compound.

It can be also defined as the numbers of electrons that an atom may lose or gain during sharing of electrons or any chemical reaction to achieve noble gas configuration.

For example – the sodium ion is represented as (Na⁺), which means that its valency is 1. Similarly, the magnesium ion is represented as (Mg²⁺) which means its valency is 2.

Rules For Writing a Chemical Formula:

 First step is to balance the valencies or charges on the ions.

One must write the name of a metal first, if the compound consists of a metal and a non-metal. For example- sodium chloride (NaCl), copper oxide (CuO), calcium oxide (CaO) etc.

The ion is enclosed in a bracket before writing the number of ion associated to it, in case of polyatomic ion. For example – (NH ₄ ) ₂ SO ₄

If there is single ion then no need to mention the ions in brackets. For example – NaOH.

Rules For Writing Formula Of Simple Compound :

 The simplest compounds, which are made up of two different elements are called binary compounds.

 Steps to write formula of a compound –

 Write the symbol of the constituent elements of the compound and their respective valencies or charges as shown below.

Crossover the valencies of the elements.

Write down the symbols with the valencies or charges which you get after crossing over.

 For example –

1.  The formula of water :

 –  The formula of the compound will be H₂O.

2.  The formula of carbon tetrachloride :

  – Formula of the compound will be CCl₄

3.  Formula of aluminium oxide :

  – Formula of the compound will be Al₂O₃

4.   Formula of ammonium sulfate:

The formula of the compound will be (NH ₄ ) ₂ SO ₄