Chemically the property of an element that makes it a metal is its ability to lose electrons to form metal cation. The tendency of a metal to lose electrons is called electropositivity. The more easily a metal loses its electrons, the more electropositive it is.
Valency
The number of electrons lost by an atom of a metal is called its valency. For example, Aluminium lose atom can lose 3 electrons to form a positive ion, So the valency of Aluminium metal is 1. Similarly, Sodium metal can lose 1 electron from its valence shell. Therefore, its valency is 1.
Trend of Electropositivity
Electropositive character of metals increases down the group with increasing atomic size. Alkali metals have large atomic size and low ionization potential values. The nucleus force on valence shell is very weak so they can lose their valence electron easily. Hence, they are highly reactive, highly electro positive, powerful reducing agents and cannot exist free in nature.
