[Bob Sacamano]

How does electron affinity increase as you move down a group on the periodic table when the effective nuclear (Z_eff) charge does not significantly change.

[renge ishyo]

You might be getting two different periodic trends crossed up with one another here. Electron affinity tends to decreases as you move down a group for row 3 and below because an extra added electron would be further away from the nucleus and would feel a lighter pull. As you move across a group from left to right the electron affinity generally increases along with Z_eff.

Be careful, there are many exceptions to these trends as you can see from the following chart which can be found in its original form with more thorough explanations at: http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch7/ie_ea.html



For instance, group 5 of the periodic table experiences a sharp decrease in electron affinity compared to the groups surrounding it due to the half filled orbital stability of atoms in this column, and the electron affinity for row 2 is actually less than those in row 3 due to the repulsion of nearby electrons that occurs when the size of the atoms is very small.