Flexi Says: Valence electrons are the outer-shell electrons of an atom. Valence electrons determine the reactivity of an atom. The number of valence electrons an element has is determined by its position in the periodic table. For s-block and p-block elements, the elements in the same group contain the same number of valence electrons, and the number of valence electrons increases by one as you go right across a period, skipping the d-block. The number of valence electrons in transition metals is somewhat more complex.
