Drops at Be to B and N to O arise from subshell shielding and electron-pair repulsion.
Practise Atomic Structure & Periodicity All Atomic Structure & Periodicity topics
Ionisation energy does not rise perfectly smoothly across a period. Boron has a lower IE1 than beryllium, because its outer electron is in a 2p orbital that is higher in energy and better shielded than the 2s electron of beryllium. Oxygen has a lower IE1 than nitrogen because removing an electron from its paired 2p orbital relieves electron-electron repulsion, whereas nitrogen has a stable half-filled p subshell. These anomalies reveal subshell energy differences and pairing effects. They are useful for teaching orbital structure.
Key ideas: Two dips in period 2 and 3: Be to B (2s is more penetrating than 2p, so 2p is easier to remove) and N to O (the paired electron in O 2p4 has repulsion, while N has stable half-filled 2p3). Be 899, B 801, N 1402, O 1314 kJ per mol. The same pattern occurs for Mg to Al and P to S.
Common mistakes and connections: Students explain the N to O drop by half-filled stability alone, while pair repulsion matters as well. Another is expecting a smooth increase. The lack of a smooth trend is evidence that subshell energies and electron pairing matter. It links to Hund's rule and penetration.
Boron is lower than beryllium (2p versus 2s) and oxygen lower than nitrogen (paired 2p electron repulsion), breaking the smooth rise. The interactive lesson shows this as a live animation or chart.
Nitrogen has an IE1 of about 1402 kJ/mol, while oxygen has 1314 kJ/mol, despite oxygen having a higher nuclear charge.
Drops at Be to B and N to O arise from subshell shielding and electron-pair repulsion.
Anomalies in Ionization Energy is a intermediate topic in Atomic Structure & Periodicity. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Atomic Structure & Periodicity first: Isoelectronic Species, Ionization Energy, Successive Ionization Energies.
The next topics in Atomic Structure & Periodicity are Electron Affinity and Electronegativity.
Yes. The interactive lesson includes a self-check question on Anomalies in Ionization Energy, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Anomalies in Ionization Energy is topic 88 of 115 in Atomic Structure & Periodicity.