Some atoms, such as metals tend to lose electrons to make the outside ring or rings of electrons more stable and other atoms tend to gain electrons to complete the outside ring. An ion is a charged particle. Electrons are negative. The negative charge of the electrons can be offset by the positive charge of the protons, but the number of protons does not change in a chemical reaction. When an atom loses electrons it becomes a positive ion because the number of protons exceeds the number of electrons. Non-metal ions and most of the polyatomic ions have a negative charge. The non-metal ions tend to gain electrons to fill out the outer shell. When the number of electrons exceeds the number of protons, the ion is negative. The attraction between a positive ion and a negative ion is an ionic bond. Any positive ion will bond with any negative ion. They are not fussy. An ionic compound is a group of atoms attached by an ionic bond that is a major unifying portion of the compound. A positive ion, whether it is a single atom or a group of atoms all with the same charge, is called a cation, pronounced as if a cat were an ion. A negative ion is called an anion, pronounced as if Ann were an ion. The name of an ionic compound is the name of the positive ion (cation) first and the negative (anion) ion second.
The valence of an atom is the likely charge it will take on as an ion. The names of the ions of metal elements with only one valence, such as the Group 1 or Group 2 elements, are the same as the names of the elements. A sodium atom that has lost the electron in its outside shell is a positive ion, sodium ion, Na+. A magnesium atom that has lost the two electrons in its outside shell is a plus two (double positive) magnesium ion, Mg++.The names of the ions of nonmetal elements (anions) develop an -ide on the end of the name of the element. Nonmetal atoms tend to GAIN electrons, so when a nonmetal atom collects an extra electron, it will become a negative ion. For instance, fluorine ion is fluoride, F-; oxygen ion is oxide, O=, a double negative ion because it gains TWO electrons; and iodine ion is iodide, I-. There are a number of elements, usually transition elements that have more than one valence and have a name for each ion, for instance, ferric ion is an iron ion with a positive three charge. Ferrous ion is an iron ion with a charge of plus two. There are a number of common groups of atoms that have a charge for the whole group. Such a group is called a polyatomic ion or radical. We suggest it is best to learn by rote the list of polyatomic ions with their names, formulas and charges and the elements with more than one valence the same way.