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AHL Extension3.1.4
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" which provide critical evidence for the existence of sub-shells.
Dip at Group 13 (Magnesium vs Aluminum)
Mg: vsAl:
Reason: The 3p sub-level is higher in energy than the 3s. The 3p electron in Al is further away and partially shielded by the 3s electrons, making it easier to remove.
Dip at Group 16 (Phosphorus vs Sulfur)
P: (all unpaired)
S: (one paired)
Reason: Spin-Pair Repulsion. The paired electron in the S p-orbital repels its partner, increasing its energy and making it easier to remove compared to the stable unpaired electron in P.
2. Successive Ionization Energies
Large jumps in successive IE indicate the removal of an electron from a new, inner principal energy level.
Example Analysis
Consider the IEs: 738, 1450, 7730, 10540 kJ/mol.
- The massive jump happens between the 2nd and 3rd IE.
- This means the 3rd electron is being removed from a core shell closer to the nucleus.
- Therefore, the element has 2 valence electrons (Group 2, Magnesium).