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In metals like lead tin gallium you find that they behave less like metals.

Metals have a tendency to share the electrons in their outer most orbitals. these are shared among neighbouring molecules and they constitute the current whene an electric potential is applied.  they are like free electrons.
In the above mentioned metasls like lead, the electrons are more tightly bonded and they are not shared among the molecules and neighbours. So this tendency of not outer orbital electrons not being shared in metals is the inert pair effect.