According to the octet rule, atoms tend to combine by gain, loss, or sharing of electrons so that the outer energy level of each atom holds or shares four pairs of electrons. This rule is really just a useful way in describing various compounds according to those that "obey" the rule and those that don't.

Metal atoms contain outermost electrons that are much more easily removed than the outermost electrons in nonmetal atoms. In a metal, each atom gives up one or more valence electrons to become a cation.

The valence electrons from all of the atoms form what has been called an "electron sea" that surrounds a network of metal cations. The freed valence electrons no longer "belong" to specific atoms.

Metallic bonding is the attraction between positive metal ions and surrounding, freely mobile electrons.

Representative metal atoms contribute their outermost s electrons or s and p electrons to the sea of free electrons. In transition metals some of the (n-1)d electrons can also join the electron sea.

Ionic bonding is the attraction between positive and negative ions. Ionic compounds are simply collections of ions held together by the force of electrostatic attraction between cations and anions.

Independent molecules do not exist in ionic compounds and in the solid-state the ions occupy fixed positions in a crystal lattice.

Example:

Hence:

(1s² 2s² 2p⁶ 3s¹) Na   +   Cl   (1s² 2p² 2p⁶ 3s ² 3p⁵)   ®   (1s² 2s² 2p⁶)   Na⁺ Cl^-   (1s² 2s² 2p ⁶ 3s² 3p⁶)  

 

Properties of ionically bound materials:

crystalline solids hard & brittle high melting points high boiling points high heats of vaporisation high heats of fusion good electrical conductors when molten poor electrical conductors when solid many are soluble in water not very good conductors of heat do not easily pass kinetic energy along to neighbours

Information provided by: http://www.bpx.cov.ac.uk