What does the trend in electron configuration have to do with effective nuclear charge?
(The question on my homework is: Use the trends in electron configuration to explain and justifity why your trend (effective nuclear charge) follows that pattern that it does.)
The effective nuclear charge is the positive charge experienced by a particular electron from the nucleus that it is surrounding. The distance of the electron from the nucleus coupled with a shielding effect from neighboring electrons causes this value to change from element to element. The general trend is that the effective nuclear charge increases as you move across a period (horizontally left to right) and it decreases as you move down a group (vertically top to bottom). The reason for this is electronic configurations. Moving across a period increases the size of the nucleus but the valence (outermost) shell of electrons remains the same. This means that the electrons are staying the same distance from a nucleus increasing in size, hence increasing the effective nuclear charge. But when you move down a group, even though the nucleus is larger, you are adding an entirely new valence shell of electrons that are further away from the nucleus, thus they have a smaller effective nuclear charge.