3.1.7 (HL)—IE discontinuities
- Syllabus
- First assessment 2025
- Objective
- 3.1.7
- Level
- HL
Ionization-Energy Discontinuities
HL onlyDiscontinuities in the general first-ionization-energy trend provide evidence for sublevel energies and electron pairing. A higher-energy p electron can be easier to remove than an s electron, and paired-electron repulsion can lower IE.
Name the sublevel and pairing evidence, not just the direction of the graph change.
The drop from Mg to Al occurs because Al loses a higher-energy 3p electron after Mg loses 3s; the drop from N to O reflects pairing repulsion in one 2p orbital. These local electronic effects explain exceptions without overturning the overall across-period rise.
Explaining IE Exceptions
HL onlyAssessment in practice
Representative question
Question 1
[Maximum number: 4]
Explain, in terms of nuclear charge, electron subshells and the shielding provided by filled electron shells, why the first ionization energy increases from Li to Be , but decreases from Be to B.
nuclear charge / number of protons increases «for both»
Li and Be «outer electrons have» same subshell/shielding electron in B lost from p-subshell whereas that in Be lost from s-subshell «outer electron in» B/p-subshell experiences greater shielding / has higher energy
Marking guidance:
Do not accept explanations invoking
distance of electrons from nucleus.
The Periodic Table Summary
Retrieve the route: locate an element from configuration, explain periodic and group trends, write oxide/reaction and oxidation-state answers, then connect incomplete d-sublevels to transition properties, ion configurations, and colours.
Check that every trend explanation names its particle-level cause, every equation is balanced, every oxidation state is a formal charge convention, and every transition colour uses absorbed/observed complementarity.