Ionic Bonding

A normal atom has a neutral charge with an equal number of protons and electrons. For example, a neutral atom of sodium has eleven protons and eleven electrons. However, since atoms are more stable with a full valence shell, an atom can gain or lose electrons to become stable. If a sodium atom loses 1 electron, it will have a full valence shell but it will no longer be neutral. The atom will now have 11 protons and 10 electrons for a net charge of +1. The neutral atom becomes charged when there are an unequal number of protons and electrons.

A neutral atom that becomes charged by gaining or losing electrons is called an ion. An atom can acquire a positive or negative charge depending on whether the number of electrons is greater or less than the number of protons in the atom. An atom with more electrons than protons, has a negative charge and is a negative ion, or anion. However, an atom with more protons than electrons, has a net positive charge and is a positive ion, or a cation. Since an electron and a proton have equal but opposite unit charges, the charge of an ion is always expressed as a whole number of unit charges and is either positive or negative. For example, calcium loses 2 valence electrons, to form a positive ion with a +2 net charge which is expressed as Ca²⁺. Alternatively, an oxygen atom gains two electrons to form a negative ion with a -2 charge, which is expressed as O²⁻. Molecules can also be ions, known as polyatomic ions. Most polyatomic ions are anions with one notable exception, the ammonium cation (NH₄⁺).

Ionic bonds form as a result of the attraction between positive and negative ions. Ionic bonding normally occurs between metal atoms and nonmetal atoms. Atoms with partially filled outer shells are unstable. To become stable, the metal atom loses one or more electrons in its outer shell, forming a positively charged ion or cation with a stable electron configuration. These electrons are then gained by the nonmetal atom, causing it to form a negatively charged ion or anion, also with a stable electron configuration. The attraction between the oppositely charged ions causes them to come together and form an ionic bond. In this example, sodium (Na) and chlorine (Cl) ions are attracted to each other in a 1:1 ratio and combine to form sodium chloride (NaCl), common table salt.

Atoms that form ionic bonds are called ionic compounds. Ionic compounds are formed from a positive cation interacting with a negative anion. The negative and positive ions form a repeating pattern. This pattern creates a crystal lattice which is orderly and three dimensional. The lattice is generally hard and brittle due to the strong bonds between the ions. The repeating pattern also provides the substance with a high melting point because of the high energy required to break all the atomic bonds. When an ionic compound is added to water, the water molecules pull apart the ions of the solid lattice. The resulting solution contains charged ions that conduct electricity.

The names of ionic compounds are written by listing the name of the positive ion followed by the name of the negative ion. Therefore, a series of rules is needed to name the positive and negative ions before we can name these compounds. Single atomic positive ions have the name of the element from which they are formed. However, since some metals form positive ions in more than one oxidation state, (Ex. Fe²⁺, Fe³⁺ ) the charge on the ion is indicated by a Roman numeral in parentheses immediately after the name of the element (Ex. iron(II), iron(III)). Polyatomic (more than two atoms) positive ions often have common names ending with the suffix "-onium" such as hydronium (H₃O⁺) or ammonium (NH₄⁺). Negative ions that consist of a single atom are named by adding the suffix "-ide" to the stem of the name of the element. The name of polyatomic negative ions usually ends in either "-ite" or "-ate." The "-ite" ending indicates a low oxidation state (nitrite ion NO₂⁻), whereas the "-ate" ending indicates a high oxidation state (nitrate ion NO₃⁻). Oxidation state shows the total number of electrons that an atom has gained or lost in order to form a chemical bond with another atom.

When an ionic compound is formed, the ions must combine in a way that the total charges of the compound equal zero. For example, let's write the correct formula for magnesium chloride. The first step is to write the formulas for the cation Mg²⁺ and anion Cl⁻. Next, drop the positive and negative signs; crisscross the superscripts so that they become subscripts and reduce when possible by finding the least common multiple. In this example, the two chlorine ions, with a total charge of -2, balance the +2 charge of the magnesium ion. The cation is always listed first before the anion, resulting in the formula MgCl₂.