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First Ionization Energy Trend

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State

First ionization energy rises across a period as Zeff increases, and falls down a group as the removed electron sits farther out.

Show

Removing an electron from a small, high-Zeff atom on a period's right takes more energy than from a larger, low-Zeff atom.

Watch for

A few systematic dips interrupt the across-period rise, tied to subshell arrangements, without overturning the Zeff-driven trend.