Ion Formation
The Octet Rule, Isoelectronic Species, and Transition Metal Ionization.
1. The Octet Rule & Energetic Stability
Atoms react to minimize their potential energy. This is typically achieved by attaining the electron configuration of the nearest Noble Gas (a full valence shell).
Cations (Positive)
Formed by metals (Low Ionization Energy).
They lose valence electrons.
Anions (Negative)
Formed by non-metals (High Electron Affinity).
They gain valence electrons.
2. Isoelectronic Species
Definition for Paper 1:
Different chemical species that possess the exact same electron configuration but distinct nuclear charges.
The Trend: Radius Decreases with Nuclear Charge
As the number of protons (Nuclear Charge, \(Z\)) increases across an isoelectronic series, the ionic radius decreases.
- Why? The number of electron shells is constant (same shielding).
- The increasing positive charge from the nucleus pulls the valence shell closer.
Worked Example: Comparing Isoelectronic Radii (The Neon Series)
Problem: Rank the following species in order of DECREASING size:
\( N^{3-}, Mg^{2+}, F^-, Na^+, O^{2-} \)
1. Check Electrons: All have 10 electrons (\(1s^2 2s^2 2p^6\)). They are isoelectronic.
2. Compare Protons (Z):
- N³⁻: 7p (Lowest Pull) → Largest Radius
- O²⁻: 8p
- F⁻ : 9p
- Na⁺: 11p
- Mg²⁺: 12p (Highest Pull) → Smallest Radius
3. Result:
\( N^{3-} > O^{2-} > F^- > Na^+ > Mg^{2+} \)
3. Transition Metal Nuance
CRITICAL EXAM TIP: 4s vs 3d
"First In, First Out" Rule: When writing the configuration for a transition metal ION, always remove the 4s electrons completely before touching the 3d electrons.
NOT [Ar] 4s² 3d⁴
Half-filled d-subshell is extra stable.
4. Required Polyatomic Ions
| Name | Formula | Charge | Note |
|---|---|---|---|
| Ammonium | NH₄⁺ | +1 | Only common polyatomic cation |
| Hydroxide | OH⁻ | -1 | Base former |
| Nitrate | NO₃⁻ | -1 | "Ate" = More Oxygen |
| Sulfate | SO₄²⁻ | -2 | Sulfuric acid salt |
Check Your Understanding
Q1. Which series is arranged in order of decreasing ionic radius?
Q2. What is the correct electron configuration for the Fe²⁺ ion?