For elements B, C, N, Li, Be, O and F, the correct order of first ionisation enthalpy is:

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Strategy:
 Follow the general periodic trend: Ionization Enthalpy () increases from left to right across a period. Account for specific anomalies caused by stable electronic configurations (filled or half-filled subshells).

Step 1: Apply general trend
Elements ( period): .
General expected order: .

Step 2: Account for anomalies
1.  vs :  () has a fully filled subshell and higher  for the  electron compared to the  electron in  (). Thus, .
2.  vs :  () has a stable half-filled subshell, while

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