What are valence electrons (be specific) ?

In chemistry, a valence electron is an outer shell electron that is associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed; in a single covalent bond, both atoms in the bond contribute one valence electron in order to form a shared pair.

Valence electrons are the electrons present in the outermost shell of an atom. You can easily determine the number of valence electrons an atom can have by looking at its Group in the periodic table ... Atoms in Groups 13 and 18 have 3 and 8 valence electrons, respectively.