why Li shows co-valent nature while it is a electropositive element …???please give me suitable reason??

This is due to the : (i) Exceptionally small size of its atom and ion, and (ii) High polarising power (i.e., charge/ radius ratio). As a result, there is increased covalent character of lithium compounds which is responsible for their solubility in organic solvents. And lithium shows diagonal relationship to magnesium .