Metal atoms find it fairly easy (in terms of energy) to lose their outer-shell or valence electrons. They have a low first ionisation enthalpy.
Li(g) ⇾ Li+(g) + e– 520 kJ mol-1
The free electron model of metallic bonding describes a metallic lattice of cations surrounded by a sea of delocalised valence electrons (this is the model you will be familiar with from GCSE).
Metals conduct electricity because when a potential difference is applied, the electrons flow causing a current. Aluminium is a better conductor than lithium because it has more valence electrons to delocalise (Al3+ + 3e–). Other properties include thermal conductivity and malleability.