A monatomic ion is an ion made of only one atom. An ion consisting of more than one atom is called a polyatomic ion. This is true even if all atoms in the polyatomic ion are the same element.
An ion is an atom or molecule that carries a positive or negative charge. This occurs when an atom gains or loses one of its valence electrons. An ion with a net positive charge is called a cation, and an ion with a net negative charge is called an anion.
Metals readily form cations by losing a valence electron, while non-metals are more likely to gain a valence electron and form an anion. When a metal and a non-metal form an ionic bond, the metal donates a valence electron to the non-metal. The resulting molecule is called an ionic compound.
When two non-metals form a compound, an ionic bond is not formed. Instead, the two atoms share their valence electrons, resulting in a covalent bond. In a polar bond, valence electrons are not shared equally between the covalent compound's atoms. This causes the two atoms to have differing charges, as in an ionic bond. In a non-polar bond, the two atoms share electrons equally, and there is no difference in charge.