Chemical Bonding
What the exam tests
Chemical Bonding tests ionic, covalent and polar covalent bonding, electronegativity, VSEPR shapes, bond angles and intermolecular forces, and links these to physical properties such as melting point, solubility and conductivity.
Core rules
- Ionic bonding: electron transfer, a lattice of ions, high melting points, conducts when molten or dissolved.
- Covalent bonding: electron sharing; a large electronegativity difference (roughly above 1.7) gives ionic character.
- VSEPR: electron pairs repel and spread out. Two regions linear (180°), three trigonal planar (120°), four tetrahedral (109.5°).
- Lone pairs repel more than bonding pairs, so NH₃ is pyramidal at about 107° and H₂O is bent at about 104.5°.
- Intermolecular forces, weakest to strongest: Van der Waals, dipole–dipole, hydrogen bonding. Hydrogen bonds need H attached to N, O or F.
- Sigma bonds are end-on overlaps; pi bonds are sideways overlaps in double and triple bonds.
Worked example
Predict the shape and bond angle of BF₃ and NF₃. Boron in BF₃ has three bonding pairs and no lone pairs: trigonal planar, 120°. Nitrogen in NF₃ has three bonding pairs and one lone pair: trigonal pyramidal, about 107°. The lone pair pushes the bonds closer together.
Traps
- Judging shape from the formula instead of counting electron pairs.
- Forgetting that a lone pair changes the shape but not the electron-pair geometry.
- Claiming hydrogen bonding occurs in every molecule containing hydrogen.
- Saying covalent compounds always conduct when dissolved; only ionising ones do.
- Confusing bond polarity with molecular polarity; CO₂ is non-polar overall.