1. The Octet Rule & Energetic Stability
Atoms react to minimize their potential energy. This is typically achieved by attaining the electron configuration of the nearestNoble Gas (a full valence shell).
Interactive: Formation of a Sodium Cation (Na⁺)
Observe how losing the valence electron affects the size of the particle.
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.
Comparing Isoelectronic Radii (The Neon Series)
Paper 2 Style\( N^{3-}, Mg^{2+}, F^-, Na^+, O^{2-} \)
1. Check Electrons: All differ but 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 |