Practice question · Multiple choice
Oxygen has a lower first ionisation energy than nitrogen despite possessing a higher nuclear charge. What accounts for this apparent contradiction, and what does it reveal about valence electrons?
Hints
- In what specific subshell configuration does nitrogen sit compared to oxygen?
- How does placing two electrons into the very same orbital alter their electrostatic interactions?
Show the answer
B. Paired electrons in a subshell repel each other, lowering removal energy
Why
Added core shielding would require new inner shells, whereas both elements share identical cores. Inner shells do not expand outward with higher Z, and increased nuclear charge generally binds electrons tighter; here, intra-orbital repulsion uniquely destabilises the pair.
Practise Periodic Properties
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