Ionic bond

Ionic bonding is a form of chemical bonding driven by the electrostatic pull between ions with opposite charges, or between atoms that have very different electronegativities. It is the main force behind ionic compounds and stands alongside covalent and metallic bonding as a fundamental bond type. Ions are atoms or groups of atoms that carry an electrostatic charge: atoms that pick up extra electrons become negatively charged anions, while those that lose electrons become positively charged cations. This electron transfer is called electrovalence, as opposed to covalence. In the simplest case, the cation is a metal and the anion is a nonmetal, but ions can also be polyatomic, like NH₄⁺ or SO₄²⁻. Put simply, an ionic bond forms when a metal gives electrons to a nonmetal so both can achieve a full valence shell.

Perfect ionic bonding—where one atom completely hands over an electron to another—doesn't actually exist; all ionic compounds have some covalent character, meaning some electron sharing. So, a bond is called "ionic" when its ionic character outweighs its covalent character—that is, when the electronegativity difference between the cation and anion is large, making the bond more polar (ionic) than a covalent bond where electrons are shared more evenly. Bonds with both ionic and covalent traits are known as polar covalent bonds.

Ionic compounds conduct electricity when melted or dissolved in a solution, but usually not when solid. They generally have high melting points, which depend on the charge of their ions: higher charges mean stronger cohesive forces and higher melting points. They also tend to dissolve in water, though stronger cohesive forces lower their solubility.

**Overview** Atoms with a nearly full or nearly empty valence shell are very reactive. Strongly electronegative atoms, like halogens, often have just one or two empty spots in their valence shell and frequently bond with other atoms or gain electrons to become anions. Weakly electronegative atoms, such as alkali metals, have few valence electrons that they can easily lose to strongly electronegative atoms. This causes weakly electronegative atoms to distort their electron cloud and form cations.

**Properties of ionic bonds** Ionic bonds are considered among the strongest of all chemical bonds, which often makes ionic compounds very stable. They have high bond energy—the average energy needed to break the bond in the gaseous state. Most ionic compounds form a crystal structure where ions sit at the corners of the crystal, called a crystal lattice. When dissolved in water or another polar solvent, ionic compounds lose this lattice and break into ions—a process called solvation. The free ions make aqueous solutions of ionic compounds good conductors of electricity. The same happens when the compounds are heated above their melting point, a process known as melting.

**Formation** Ionic bonding can come from a redox reaction when an element (usually a metal) with low ionization energy gives up some electrons to reach a stable electron configuration, forming cations. Another element (usually a nonmetal) with high electron affinity accepts those electrons to also become stable, turning into an anion. For s-block and p-block elements, the stable configuration is typically that of a noble gas; d-block and f-block elements have their own particular stable configurations. The electrostatic attraction between anions and cations builds a solid with a crystallographic lattice where ions stack in an alternating pattern. In such a lattice, you usually can't pick out discrete molecular units, so the compounds aren't molecular. However, the ions themselves can be complex, forming molecular ions like the acetate anion or ammonium cation.

For example, common table salt is sodium chloride. When sodium (Na) and chlorine (Cl) combine, each sodium atom loses an electron to become a Na⁺ cation, and each chlorine atom gains an electron to become a Cl⁻ anion. These ions attract each other in a 1:1 ratio to form NaCl: Na + Cl → Na⁺ + Cl⁻ → NaCl To keep charge neutral, strict ratios between anions and cations are followed, so ionic compounds generally obey stoichiometry even though they aren't molecular. For compounds that are transitional to alloys and have mixed ionic and metallic bonding, this may not hold. Many sulfides, for instance, form non-stoichiometric compounds.

Many ionic compounds are called salts because they can also form from the neutralization reaction of an Arrhenius base, like NaOH, with an Arrhenius acid, like HCl: NaOH + HCl → NaCl + H₂O The salt NaCl is then said to consist of the acid rest Cl⁻ and the base rest Na⁺.

Removing electrons to form the cation is endothermic, raising the system's overall energy. There may also be energy changes from breaking existing bonds or adding more than one electron to form anions. However, when the anion accepts the cation's valence electrons and the ions attract each other, lattice energy is released, lowering the system's overall energy. Ionic bonding will only happen if the overall energy change for the reaction is favorable. Usually, the reaction is exothermic, but the formation of mercuric oxide (HgO), for example, is endothermic.