An ionic bond can be formed after two or more atoms give up (or gain) electrons, so as to become ions. This type of bonding occurs between metals and non-metals. The atom that loses electron(s) is usually a metal whilst the atom that gains is usually a non-metal. Ions of opposite charge will attract one another, thus creating an ionic bond. Such bonds are stronger than hydrogen bonds, but similar in strength to covalent bonds.
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It only occurs if the overall energy change for the reaction is favourable (the bonded atoms have a lower energy than the free ones). The larger the energy change the stronger the bond.\\
Pure ionic bonding doesn't actually happen with real atoms. All bonds have a small amount of covalency. The larger the difference in electronegativity the more ionic the bond.
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The diagram above shows the electron configurations of lithium and fluorine. Note that lithium has one electron in its outer shell. This electron is held rather loosely (because the ionisation energy is very different in the two ions). Note also that fluorine has 7 electrons in its outer shell. If the electron moves from lithium to fluorine each ion acquires the configuration of a noble gas. The bonding energy (from the electrostatic attraction of the two oppositely charged ions) is large enough (negative value) that the overall bonded state energy is lower than the unbonded state.
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Impression of two ions (for example [Na]+ and [Cl]-) forming an ionic bond. Electron orbitals generally do not overlap (ie. molecular orbitals are not formed), because each of the ions reached the lowest energy state, and the bond is based only (ideally) on the electrostatic interactions between positive and negative ions.
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