Here's why:

* Valence electrons are the electrons in the outermost shell of an atom, which are involved in chemical bonding.

* Group number in the periodic table generally tells us the number of valence electrons an atom has.

* Group 16 elements are two columns to the right of the noble gases (group 18). Since noble gases have a full outer shell (8 valence electrons), group 16 elements are two electrons short.

Therefore, group 16 elements have 6 valence electrons.