The first ionization energy (in kJ/mol) of Na, Mg, Al and Si respectively are:

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Strategy:
 Rank Period 3 members (Na, Mg, Al, Si) by . Account for the anomaly where removing an electron from a -subshell (Group 13) is easier than from a fully-filled -subshell (Group 2).

Step 1: Comparison
- Sodium:  () - Lowest due to largest size.
- Aluminum:  ().
- Magnesium:  (). (Anomaly: Mg  Al)
- Silicon:  ().

Step 2: Order
.

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