Question

why are metals called electropositive elements whereas non-metals are called electronegative elements?

Answer 1

An electropositive element is one that has tendency to lose electrons and form a cation. Metals like Na, K, Mg, Ca, Fe, Zn easily lose electrons as most of them have a few electrons in their valence shell.
An electronegative element is one which attracts electrons towards itself and is capable to gain electrons and form anions. Non-metals like F, Cl, O are capable to gain electrons and form anions.