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.

The correct answer is that they have the same number of valence electrons - 7!

Valence electrons are electrons on the outside shell of the atom, and they are the part of the atom that interacts with other atom and participates in the forming of complex molecules. 

The same number of valence electrons means that they interact in similar ways with other atoms, and that's why they are organised into the same group.