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

number of valence electrons

Explanation:

All of the elements in group 17, also known as group 7A, have 7 valence electrons.

No two elements have the same atomic number nor atomic mass in elemental form.

Halogens may have any number of isotopes.