How do we know whether an element is a metal or a non metal from its atomic number? 1;f the valence electron number is either 1,2 or 3 then it is a Non Metal and if the valence electron number is either 4,5 or 6 then it is a Metal 2:If the valence electron number is either 1,2 or 3 then it is a Metal and if the valence electron number is either 5,6 or 7 then it is a Non Metal 3:if the valence electron number is either 1,2 or 3 then it is a Metal and if the valence electron number is either 4,5 or 6 then it is a Non Metal

Explanation:

The valence number represents the number of electrons in the outermost shell of the atom. Interestingly, in a group 1 -8 periodic tale, the number represents the valence electrons.

Thus means that group 3 electrons have a valency of 3; that is all group 3 elements have 3 electrons in their outermost shell.

Metals easily lose electrons rather than gain electrons while non metals gain electrons.

In the periodic table; groups 1, 2, 3 represents metals and from group 4 upwards we have non metals.

Going through the options now, the correct option is;

3:if the valence electron number is either 1,2 or 3 then it is a Metal and if the valence electron number is either 4,5 or 6 then it is a Non Metal

Group  7 elements ( Valency of 7) are also non metals.