What is the relationship between the atomic numbers and ionic radii of the elements in the group 7a

The ionic radii increase as the atomic numbers increase.

Explanation:

As we go down a Group, we are adding one more electron shell to the ion.

Each successive shell is bigger than the one before it, so the ionic radius should increase with increasing atomic number.

Here is a table of the ionic radii of the X⁻ ions.

Element Z r/pm

     F          9   133

     Cl       17    181

     Br      35   196

     I         53  220

     At      85  227

However, the relation between r and Z is not linear (see graph).

All we can say is the ionic radius increases as the atomic number increases.