what is a polyatomic ion?how it is different from an ion,metallic and non metallic elements?

A polyatomic ion is also known as a molecular ion.

 These are charged species (ion) composed of two or more atoms covalently bonded or of a metal complex that can be considered as acting as a single unit in the context of acid and base chemistry or in the formation of salts. The prefix "poly-" means "many," in Greek, but even ions of two atoms are commonly referred to as polyatomic.

 

Examples:  A hydroxide ion is made of one oxygen atom and one hydrogen atom: its chemical formula is OH^-, similarly SO₄^2-, NH₄⁺.

 

Ion: The atoms or groups of atoms which carry a positive charge or a negative charge are known as ions.

Metallic element: Elements have a tendency to lose their valence electrons and form a positive ion.

Eg. Na⁺, Mg⁺²

Non-metallic element: Elements have a tendency to gain electrons and form a negative ion.

Eg. Cl^-, F^-