🟣 This is Higher Level (HL) content.
📘 IB Understanding
First ionisation energy (IE₁) is the minimum energy required to remove one mole of electrons from one mole of gaseous atoms in their ground state:
M(g) → M⁺(g) + e⁻
The general trend across Period 3 is an increase in IE₁ due to increasing nuclear charge while electron shielding remains relatively constant. However, there are two distinct discontinuities that provide empirical proof for the existence of energy sublevels.
1st Ionisation Energy across Period 3
Anomaly 1: Mg → Al
Mg: [Ne] 3s². Electron removed from 3s subshell
Al: [Ne] 3s² 3p¹. Electron removed from 3p subshell
The 3p subshell is slightly higher in energy and further from the nucleus than the 3s subshell. The 3p electron is therefore easier to remove, despite Al having a higher nuclear charge.
Anomaly 2: P → S
P: [Ne] 3s² 3p³. Three unpaired electrons in 3p (↑ ↑ ↑)
S: [Ne] 3s² 3p⁴. One orbital now has a paired electron (↑↓ ↑ ↑)
The paired electron in S experiences spin-pair repulsion (inter-electronic repulsion) within the same orbital. This raises its energy and makes it easier to remove, so S has a lower IE₁ than P.
Electron Configuration Summary
| Element | Config | IE₁ (kJ mol⁻¹) | Explanation |
|---|---|---|---|
| Na | 3s¹ | 496 | One 3s electron, easily removed |
| Mg | 3s² | 738 | Full 3s, higher Zeff |
| Al | 3s² 3p¹ | 577 | 3p electron higher energy → easier to remove |
| Si | 3s² 3p² | 786 | Increasing Zeff |
| P | 3s² 3p³ | 1012 | Half-filled 3p, no pairing |
| S | 3s² 3p⁴ | 1000 | Spin-pair repulsion in paired orbital |
| Cl | 3s² 3p⁵ | 1251 | Increasing Zeff dominates |
| Ar | 3s² 3p⁶ | 1521 | Full outer shell, highest Zeff |
⚠️ Common IB Mistake
Do NOT explain the anomalies using "special stability" of half-filled or full subshells. The IB requires explanations based on orbital energy levels (3s vs 3p) and electron-electron repulsion (spin-pairing). This is a key distinction from some other syllabuses.
⚠️ Exam Tip
These anomalies mirror Period 2 (Be→B and N→O). You must be able to explain both periods using electron configurations and orbital diagrams. Examiners may ask you to compare the two periods directly.