What are polyatomic ions and how are they represented in formulae?

Polyatomic ions are ions that consist of two or more atoms covalently bonded that act as a single unit with a net charge.

Polyatomic ions, also known as molecular ions, are a group of atoms that are covalently bonded together and carry an overall charge. This means that they behave as a single unit in a chemical reaction or when interacting with other ions and molecules. They are represented in chemical formulae as a group of symbols of the constituent atoms, enclosed in brackets if necessary, with the charge written as a superscript on the outside.

For example, the polyatomic ion sulfate consists of one sulfur atom and four oxygen atoms, and carries a charge of -2. It is represented in chemical formulae as SO4^2-. If a formula requires more than one of these ions, the entire formula for the ion is enclosed in brackets with the subscript indicating the quantity. For instance, aluminium sulfate, which contains two sulfate ions, is written as Al(SO4)2.

Polyatomic ions can be made up of different types of atoms. For example, the ammonium ion is made up of one nitrogen atom and four hydrogen atoms, and carries a charge of +1. It is represented in chemical formulae as NH4^+.

Understanding polyatomic ions is crucial in chemistry because they often form part of important compounds. They are involved in many chemical reactions and are commonly found in nature. For instance, the nitrate ion (NO3^-) is a key component of fertilisers and the carbonate ion (CO3^2-) is found in limestone and chalk.

Remember, the charge on the polyatomic ion is the charge of the entire group of atoms, not just one of the atoms. The atoms within the ion are held together by covalent bonds, but the ion as a whole can form ionic bonds with other ions.