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 -
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.