Sodium and chlorine bonding ionically to form sodium chloride. Sodium loses its outer electron to give it a noble gas electron configuration, and this electron enters the chlorine atom exothermically. The oppositely charged ions are then attracted to each other, and their bonding releases energy. The net transfer of energy is that energy leaves the atoms, so the reaction is able to take place.

An ionic bond (or electrovalent bond) is a type of chemical bond that can often form between metal and non-metal ions (or polyatomic ions such as ammonium) through electrostatic attraction. In short, it is a bond formed by the attraction between two oppositely charged ions.

The metal donates one or more electrons, forming a positively charged ion or cation with a stable electron configuration. These electrons then enter the non metal, causing it to form a negatively charged ion or anion which also has a stable electron configuration. The electrostatic attraction between the oppositely charged ions causes them to come together and form a bond.

For example, common table salt is sodium chloride. When sodium (Na) and chlorine (Cl) are combined, the sodium atoms each lose an electron, forming a cation (Na⁺), and the chlorine atoms each gain an electron to form an anion (Cl^-). These ions are then attracted to each other in a 1:1 ratio to form sodium chloride (NaCl).

Na + Cl ? Na⁺ + Cl^- ? NaCl

Electron configurations of lithium and fluorine. Lithium has one electron in its outer shell, held rather loosely because the ionization energy is low. Fluorine carries 7 electrons in its outer shell. When one electron moves from lithium to fluorine, each ion acquires the noble gas configuration. The bonding energy from the electrostatic attraction of the two oppositely-charged ions has a large enough negative value that the overall bonded state energy is lower than the unbonded state

Pure ionic bonding is not known to exist. All ionic compounds have a degree of covalent bonding. The larger the difference in electronegativity between two atoms, the more ionic the bond. Ionic compounds conduct electricity when molten or in solution. They generally have a high melting point and tend to be soluble in water.

Polarization effects

Ions in crystal lattices of purely ionic compounds are spherical; however, if the positive ion is small and/or highly charged, it will distort the electron cloud of the negative ion. This polarization of the negative ion leads to a build-up of extra charge density between the two nuclei, i.e., to partial covalency. Larger negative ions are more easily polarized, but the effect is usually only important when positive ions with charges of 3+ (e.g., Al³⁺) are involved. However, 2+ ions (Be²⁺) or even 1+ (Li⁺) show some polarizing power because their sizes are so small (e.g., LiI is ionic but has some covalent bonding present).
(Note that this is not the ionic polarization effect which refers to displacement of ions in the lattice due to the application of an electric field

Ionic structure

Ionic compounds in the solid state form three dimensional lattice structures, (see ionic crystal). The two principal factors in determining the form of the lattice are the relative charges of the ions and their relative sizes. Some structures are adopted by a number of compounds, for example the rock salt, sodium chloride, structure is adopted by many alkaline earth halides and binary oxides such as MgO.

Ionic versus covalent bonds

In an ionic bond, the atoms are bound by attraction of opposite ions, whereas, in a covalent bond, atoms are bound by sharing electrons. In covalent bonding, the molecular geometry around each atom is determined by VSEPR rules, whereas, in ionic materials, the geometry follows maximum packing rules.

Electrical conductivity

Ionic substances in solution conduct electricity because the ions are free to move and carry the electrical charge from the anode to the cathode.
Ionic substances conduct electricity when molten for the same reason i.e. that ions are free to move.
Some ionic compounds conduct electricity when solid, this is due to migration of ions under the influence of an electric field. (see Fast ion conductor)

Substances in ionic form

See also

• Coulomb's law
• Linear combination of atomic orbitals
• Hybridisation
• Chemical polarity

External links

• Ionic bonding tutorial
• Video on ionic bonding

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