Fifth ionization energy highest for core electron removal from innermost shell

Steps:

• Determine valence electrons: C and Si have 4 (group 14), N and P have 5 (group 15).
• For C and Si, first 4 IEs remove valence electrons, fifth targets core electrons.
• For N and P, fifth IE removes a valence electron from p orbital.
• Core removal from C's 1s shell requires more energy than Si's 2p or valence p orbitals due to nuclear proximity.

Why A is correct:

• Carbon's fifth IE removes a 1s core electron, tightly bound by the nucleus per Coulomb's law, demanding highest energy.

Why the others are wrong:

• B. Nitrogen's fifth IE removes a 2p valence electron, less bound than core.
• C. Phosphorus's fifth IE removes a 3p valence electron, shielded and farther from nucleus.
• D. Silicon's fifth IE removes a 2p core electron, less tightly held than carbon's 1s.

Final answer: A