As we move down the column of Group 7 elements, their reactivity decreases. In each, the outer shell is further from the nucleus and the incoming electron is not as easily attracted to it.

Rhenium was the first of these two new group-7-elements to be discovered — in 1925 — and accepted by the scientific community. In the course of an arduously long extraction, Walter Noddack ...

As we move down the column of Group 7 elements, their reactivity decreases. In each, the outer shell is further from the nucleus and the incoming electron is not as easily attracted to it.