The first ionization enthalpy of Na, Mg and Si, respectively, are 496, 737 and 786 kJ mol. The first ionization enthalpy (kJ mol) of Al is:

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Strategy:
 Utilize the known  anomaly between Group 2 and Group 13 in the same period. The -orbital electron is further from the nucleus and more shielded than the -orbital electrons.

Step 1: Analyze configurations
- Magnesium (, Z=12):  ().
- Aluminum (, Z=13): .
- Silicon (, Z=14):  ().

Step 2: Compare energy values
Removing a  electron from Aluminum is easier than removing a  electron from Magnesium's filled subshell. Thus,  must be lower than .
Among the options,  is the standard value for

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