A complex ion is a tiny particle of matter called a molecule which carries either a positive or negative charge. Complex ions are made of a group of atoms bonded together into a single, larger particle. Ions are simply defined as molecules that carry either a positive or negative charge, meaning there are more total protons than electrons, or vice versa. Complex ions must be made of more than one type of atom and can be broken down into two or more smaller ions. All complex ions contain at least one metal or positively charged atom bonded to one or more negatively charged ions.
A bit of background about the structure of atoms is helpful to understanding ions. An atom is the tiniest unit of a pure element that can exist. For example, if a piece of pure gold is broken down again and again until the smallest microscopic speck is reached, the result is a single atom of gold. This atom would be made of 79 positive protons clustered into a center nucleus and surrounded by a cloud of 79 negative electrons. If the gold atom gives up one of its negative electrons, it will be left with a charge of positive one and is then be considered an ion.
In chemistry, a metal is an element or type of atom that carries a positive charge. When looking at the periodic table, the elements on the left and in the middle are metals, except for hydrogen. Complex ions occur when an electron from one of these metals bonds to another atom or molecule.
The general structure of complex ions is a centrally located atom of a metal surrounded by and bonded to other ions. Together, the metal and surrounding ions form a single larger piece called a molecule. The total charge on the molecule is either positive or negative, but not neutral.
One common complex ion is the sulfate ion. This ion is composed of one sulfur atom surrounded by four oxygen atoms. The sulfur atom is acting as the metal, not because it is shiny, strong, and malleable like everyday metals, but because sulfur gives away electrons to the surrounding oxygen atoms, leaving it with a positive charge. The sulfate ion carries a charge of negative two because it has two more total electrons than protons, and having a charge is the definition of an ion.