2.3.2 Factors Affecting Bond Strength
Why some metals melt at 63°C and others at 3400°C.
The Rule of Strength
Strength ∝ (Number of Delocalized Electrons × Charge of Cation) / Ionic Radius
Essentially, smaller, highly charged ions with more delocalized electrons form stronger metallic bonds. This is often referred to as Charge Density.
Trend 1: Across Period 3 (Na → Mg → Al)
| Meta | Charge | Electrons In Sea | Ionic Radius (pm) | Melting Point (°C) |
|---|---|---|---|---|
| Sodium (Na) | +1 | 1 | 102 | 98 |
| Magnesium (Mg) | +2 | 2 | 72 | 650 |
| Aluminum (Al) | +3 | 3 | 54 | 660 |
Explanation: Melting point increases drastically from Na to Al.
- Charge Increases: The attraction between the nucleus and the sea is stronger.
- Delocalized Electrons Increase: There are more "gluing" agents per cation (1 vs 2 vs 3).
- Radius Decreases: The delocalized electrons are closer to the nucleus, strengthening the electrostatic attraction.
Trend 2: Down Group 1 (Li → Cs)
As you go down the group, melting point decreases.
- Constant Charge: All Group 1 metals form +1 ions with 1 delocalized electron.
- Increasing Radius: The cations get larger due to more occupied electron shells.
- Weaker Attraction: The distance between the positive nucleus and the delocalized electrons increases, weakening the electrostatic force.
Lithium (Li)180°C
Sodium (Na)98°C
Potassium (K)63°C
Cesium (Cs)29°C
Radius Increases, Strength Decreases