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.
First Ionisation Energies of Period 3 Elements
Anomaly 1: Mg → Al
Magnesium to Aluminium (Mg → Al): 3s vs 3p Subshell
Mg (\([\text{Ne}]\,3\text{s}^2\)): Electron removed from \(3\text{s}\) subshell.
Al (\([\text{Ne}]\,3\text{s}^2\,3\text{p}^1\)): Electron removed from \(3\text{p}\) subshell.
The \(3\text{p}\) subshell is higher in energy and partially shielded by the \(3\text{s}^2\) pair. Consequently, less energy is required to ionise Aluminium than Magnesium despite the higher nuclear charge.
Anomaly 2: P → S
Phosphorus to Sulfur (P → S): Spin-Pairing Repulsion
P (\([\text{Ne}]\,3\text{s}^2\,3\text{p}^3\)): Half-filled \(3\text{p}\) subshell with 3 unpaired electrons (\(\uparrow\, \uparrow\, \uparrow\)).
S (\([\text{Ne}]\,3\text{s}^2\,3\text{p}^4\)): One paired \(3\text{p}\) orbital (\(\uparrow\!\downarrow\, \uparrow\, \uparrow\)).
The paired electron in Sulfur experiences spin-pair repulsion within the doubly occupied orbital, raising its energy and lowering the first ionisation energy below that of Phosphorus.
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 | 578 | 3p electron higher energy → easier to remove |
| Si | 3s² 3p² | 789 | 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 |
Avoid "Half-Filled Octet Stability" Fallacy
Do NOT explain these anomalies in IB exams using "special stability of half-filled subshells". IB mark schemes strictly reward explanations based on relative orbital energy levels (3s vs 3p) and spin-pairing inter-electronic repulsion.
Comparing Period 2 and Period 3 Anomalies
Period 3 anomalies directly mirror Period 2 (\(\text{Be}\rightarrow\text{B}\) and \(\text{N}\rightarrow\text{O}\)). Be prepared to write electron configurations and orbital box diagrams for both periods, as exam questions frequently ask students to compare the two periods directly.
Study this topic on the go
Get active recall flashcards, notes, and topic quizzes in ChemEasy, or build your revision schedule with ChemPlan IB.