Does the electron shielding increase or decrease as you go down a group (for atomic radii)?
Shielding increases as you go down a group.
Electrons in higher energy levels experience a greater shielding effect than electrons in lower energy levels. This is due to the fact that while they are attracted to the positively charged nucleus, they are repelled by the negatively charged electrons in lower energy levels. Remember that like charges will repel.
This means that for every additional energy level, there are more and more electrons in lower energy levels that will repel the electrons in the highest energy level of an atom.
This means that the outer electrons experience an attraction to the positive nucleus that is much weaker than electrons in lower energy levels. This is why elements that are lower in a group will lose electrons much more easily than elements that are higher in the group.
You might find this video helpful in understanding trends of the periodic table.
Hope this helps!