Ionic bonds are formed by the transfer of one or more valence electrons between atoms, typically between metals and nonmetals. The transfer of electrons allows the atoms to effectively achieve the much more stable electron configuration of the noble gases. An example of ionic bond formation can be seen in the animation below, in which sodium donates a valence electron to fluorine.
In ionic bond formation, the atom that donates electron density is known as the cation, while the atom that gains electron density is known as the anion.
Ionic compounds can be described using chemical formulas, which represent the ratios of interacting elements that are found in the ionic solid or salt. Ionic solids are typically represented by their empirical formulas. In written and formula form, the cation is present first and is followed by the anion, with the suffix of the anion changed to -ide. In formula notation, the elements are represented by their chemical symbols followed by numeric subscripts that indicate the relative ratios of the constituent atoms. The molecular formula for an ionic compound can be determined by satisfying two conditions:
- First, the charge on the constituent atoms can be determined based on the transfer of valence electrons necessary in order to satisfy the octet rule.
- Second, the cations and anions are combined in a ratio that produces a electrically neutral product compound.
For example, in the reaction of calcium and chlorine, the product compound is called calcium chloride. It is composed of Ca2+ cations and Cl- anions, as those charges produce filled valence shells. Finally, its molecular formula is written as CaCl2, the neutral combination of these ions.