Advanced Trends & Discontinuities
Evidence for sub-shells through Ionization Energy anomalies.
1. Discontinuities in First IE
The general increase in IE across Period 3 has two important "dips" providing critical evidence for sub-shells.
Dip at Group 13 (Mg vs Al)
Mg: \([Ne]3s^2\) vs Al: \([Ne]3s^2\,3p^1\)
Reason: The 3p sub-level is higher in energy than 3s. The 3p electron is partially shielded by the 3s electrons, making it easier to remove.
Dip at Group 16 (P vs S)
P: \(3p^3\) (all unpaired)
S: \(3p^4\) (one paired)
Reason: Spin-Pair Repulsion. The paired electron repels its partner, making it easier to remove than the stable half-filled set in P.
2. Successive Ionization Energies
Large jumps in successive IE indicate removal of an electron from a new, inner principal energy level.
Example Analysis
IEs: 738, 1450, 7730, 10540 kJ/mol
- The massive jump is between the 2nd and 3rd IE.
- The 3rd electron is from a core shell closer to the nucleus.
- Therefore: 2 valence electrons → Group 2 (Magnesium).