Answer
As one proceeds down the group 7A elements, the first ionization energy would be smaller and smaller, due to the constant increase in atomic radii of the group 7A elements, making the outermost electrons more likely to be further from the nucleus and experience less attraction as a result.
Work Step by Step
The first ionization energy depends on two variables: the size of the atom and the effective nuclear charge of the atom on the outermost electrons.
As we go down in a group, though the effective nuclear charge does increases slightly, the atom size increases much more and dominates the increase of the effective nuclear charge. Therefore, the increasing size of the atom means the electrons are more likely to be further from the nucleus and experience less attraction from the nucleus. The first ionization energy, as a result, would be smaller as we go down in a group.
So, as one proceeds down the group 7A elements, the first ionization energy would be smaller and smaller. This is due to the constant increase in the atomic radii of the elements in group 7A as we go down, making the outermost electrons more likely to be further from the nucleus and undergo less and less attraction from the nucleus and easier to be removed as a result.
