Short on time? This page is the recap. For explanations, graphs and worked examples, use the full study guide.
Recap
- A potential energy curve plots energy (kJ mol⁻¹) against internuclear distance (pm). Zero = atoms far apart and not interacting.
- As atoms approach, attraction (each nucleus for both atoms’ electrons) lowers the energy. Too close, repulsion (nucleus–nucleus, electron–electron) raises it steeply.
- The minimum gives the equilibrium bond length. The depth of the minimum below zero is the bond energy, the energy needed to separate the atoms.
- Covalent bonds: bigger atomic cores give longer bonds; longer bonds are usually weaker. Higher bond order gives shorter bonds with larger bond energies.
- Ionic interactions: Coulomb’s law. Attraction is proportional to each ion’s charge and grows as the distance between ion centres shrinks.
Key relationships
| Change | Bond length | Bond energy | Example |
|---|---|---|---|
| Larger atoms (down a group) | longer | smaller | C–F 134 pm, 485 → C–I 213 pm, 240 kJ mol⁻¹ |
| Higher bond order | shorter | larger | C–C 154 pm, 347 → C≡C 120 pm, 839 kJ mol⁻¹ |
| Larger ion charges | — | stronger attraction | Mg²⁺O²⁻ > Na⁺F⁻ |
| Smaller ions (same charges) | — | stronger attraction | Na⁺F⁻ > Na⁺Cl⁻ |
Coulomb’s law: F ∝ q₁q₂ / r², with r ≈ sum of the ionic radii.
Assumptions
- Tabulated bond energies are averages over many compounds.
- Comparing ions with q₁q₂ / r² treats them as point charges in contact; it ranks, it does not give a measured energy.
Mistakes to avoid
- Quoting bond energy as negative. The minimum is negative; the bond energy (energy to break) is positive.
- Saying bond breaking releases energy. It always absorbs energy.
- Reading the bond length at the zero crossing instead of at the minimum.
- Drawing curves that do not level off at zero at large distances.
- Assuming a double bond is twice as strong as a single bond.
- Using only charge or only size when ranking ionic attractions.
- Missing units or scale on a sketched graph.
Quick self-check
- A curve has its minimum at 120 pm and −500 kJ mol⁻¹. State the bond length and bond energy. (120 pm; 500 kJ mol⁻¹)
- Which bond is shorter, C=C or C–C? (C=C)
- Which pair attracts more strongly, K⁺ with F⁻ or K⁺ with I⁻? (K⁺ with F⁻: same charges, but F⁻ is smaller.)
Next: practice questions.