There are no free electrons, ionic crystals are insulators.Similarities:
(a) Ionic and metallic crystals both have electrostatic force of attraction:
In ionic crystals, this force is between oppositely charged ions whereas in metals, this force is betweem the valence electrons and the kernels.
(b) In both cases, bonds are non-directional
Differences:
(a) In ionic crystals, ions are not free to move. So they don't conduct electricit in solid state. They are only conductors in molten state or aqueous solutions. In metals, valence electrons can move. Hence, they can conduct electricity in solid state.
(b) Ionic bonds are strong whereas metallic bonds may be weak or strong.
Ionic crystals have at least two atoms in their base which are ionized. Charge neutrality demands that the total charge in the base must be zero; so we always need ions with opposing charge.
The binding between the ions is mostly electrostatic and rather strong (binding energies around 1000 kJ/mol); it has no directionality.
Ionic crystals thus can be described as an ensemble of hard spheres which try to occupy a minimum volume while minimizing electrostatic energy at the same time (i.e. having charge neutrality in small volumes, too).
A metal is a lattice of positive metal 'ions' in a 'sea' of delocalised electrons.
Metallic bonding refers to the interaction between the delocalised electrons and the metal nuclei.
The physical properties of metals are the result of the delocalisation of the electrons involved in metallic bonding.
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