The first ionization enthalpies of Be, B, N and O follow the order:

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Strategy:
 First ionization enthalpy () generally increases across a period but is interrupted by stable electronic configurations: filled  (Group 2) and half-filled  (Group 15).

Step 1: Analyze configurations
-  (): Removing one electron gives stable filled .
-  (): Removing an electron breaks a stable filled subshell.
-  (): Removing one electron gives stable . High electron repulsion in  makes it easier than N.
-  (): Removing an electron breaks a stable half-filled subshell.

Step 2: Apply

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