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.)

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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.

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