Ions and Compounds


An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass ("go") between electrodes in a solution, when an electric field is applied. It is from Greek ιον, meaning "going." An ion may consist of single charged atom or a group of atoms. Ions with single charged atom are known as monoatomic ions and those with a group of atoms are known as polyatomic ions.


Atoms combine with each other in two broad general ways -

  • Molecular Compounds - sharing of electrons between atoms.
  • Ionic Compounds - transfer of electrons from one atom to another.

Writing chemical formula of a compound

Just like atoms have a symbol compounds too have symbolic representations known as its formulae. The valency - combining capacity of one atom with other - helps us to find the chemical formula for that compound. Valency is a measure of the number of bonds formed by an atom of a given element.

Valency of some ions

Rules for writing chemical formula

  • The valency of charges on the ion must balance.
  • When a compound consists of a metal and a non-metal, the name or symbol of the metal is written first - e.g. sodium chloride (NaCl).
  • In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio.

Worksheets and in-class activities

  • Coming soon...

Teaching and Learning aids

Assignments and Project ideas

Further reading