Question

the elements in group 17 of the periodic table are all called halogens. all halogens have the same
atomic number.
atomic mass.
number of possible isotopes.
number of valence electrons.

Explanation:

Elements in the same group of the periodic table have the same number of valence electrons. Halogens in group 17 each have 7 valence electrons. Atomic number, atomic mass, and number of possible isotopes vary among different halogen elements.

Answer:

number of valence electrons