An ion is an atom or group of atoms with a positive or negative charge. An atom is neutral because it has equal numbers of protons and electrons. An atom becomes an ion by gaining or losing electrons. The number of protons in the nucleus never changes when an ion forms.
If an atom loses electrons, it has more protons than electrons, so it forms a positive ion, also called a cation. A sodium atom has 11 protons and 11 electrons. The sodium ion has 11 protons and 10 electrons, so it has an overall charge of 1+. If an atom gains electrons, it has more electrons than protons, so it forms a negative ion, also called an anion. A chlorine atom has 17 protons and 17 electrons. The chloride ion has 17 protons and 18 electrons, so it has a charge of 1-.
Some ions are made from a group of atoms joined together with an overall charge. The hydroxide ion is one oxygen atom and one hydrogen atom with a 1- charge. The sulfate ion is one sulfur atom and four oxygen atoms with a 2- charge.