A bond in which metal atoms are held together as a result of an attractive force is called a metallic bond.
Two common models which explain the formation of metallic bond are electron sea model and the band model.
According to the electron sea model, the atoms constituting a metal possess several vacant orbitals in their valence shells. The characteristic properties of metals which are explained with the help of electron sea model are metallic lustre electrical conductivity, thermal conductivity, malleability, ductility, tensile strength and hardness.
Band model is based on molecular orbital theory. Molecular orbital theory states that the metallic bond results from the attraction between the delocalised electrons present in the closely packed molecular orbitals bands and the metal atoms. The band formed from atomic orbitals with lower energy is called valence band. The band formed from atomic orbitals with higher energy is called conduction band.
van der Waals forces are very short lived inter-molecular attractive forces which exist between all kinds of atoms, molecules and ions when they are brought sufficiently close to one another. The different types of van der Waals forces are dipole-dipole interactions, ion-dipole interactions, dipole-induced dipole interactions and instantaneous dipole-induced dipole interactions. The factors affecting the strength of the van der Waals forces are temperature, pressure, molecular size and number of electrons in atoms and molecules. van der Waal’s forces can be used for explaining melting and boiling point of substances, hardness of solids, liquefaction of gases, deviation of real gases from ideal gas behaviour and Joule-Thomson effect.
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