Question

It's it true that those elements which have 1,2,3 in the outermost shell are metal and 4,5,6,7,8 in the outermost shell are non metal???

Answer 1

Elements that have valency 1,2,3 tend to be metals(they lose electron to attain stability.

Elements having 5,6,7 tend to be non metals(gain electrons to attain stability.

Elements having 4 valence electron either gain or lose electron
But In the case of carbon
It has 4 electrons they do not lose or gain electron,they share electrons(they form covalent bonds)

Elements having 8 valence electrons have already acquired octet structure so they do not react with other metal or non metals

I think I have justified my answer for ur question

Answer 2

$\large\color{red}\textsf{Hey Dude!}$

Yes it is true!

As we know that metals have the tendency to lose electron and therefore acquires a positive charge.
So elements having valance electrons 1, 2 and 3 will loose their electrons and will thus acquire a positive charge, which means that the elements are METALS.

And also the elements having valance electrons 4, 5, 6 and 7 will acquire a negative charge by gaining electrons, which inturn symbolize that these elements are NON METALS.