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

They have the same number of valence electrons

Explanation:

The groups on the periodic table are the vertical lines of groups. In group 18 the elements are known as noble gases. They have all 8 valence electrons. As you go to the left the groups have one less valence electron. Group 17 has 7 valence electrons.