ZIMSEC A Level · Chemistry · Form 5
Bonding types
Ionic (electrostatic attraction between ions), covalent (shared pairs, including dative bonds where both electrons come from one atom, e.g. NH₄⁺) and metallic.
Electronegativity and polarity
Electronegativity increases across a period and up a group (F is highest). Unequal sharing makes a bond polar; symmetrical molecules (CO₂, CCl₄) are non-polar overall.
Shapes (VSEPR theory)
Electron pairs repel and arrange themselves as far apart as possible; lone pairs repel more than bonding pairs.
- 2 bonding pairs: linear, 180° (CO₂, BeCl₂).
- 3: trigonal planar, 120° (BF₃).
- 4: tetrahedral, 109.5° (CH₄); 3 bonding + 1 lone pair: pyramidal, 107° (NH₃); 2 bonding + 2 lone pairs: bent, 104.5° (H₂O).
- 5: trigonal bipyramidal (PCl₅); 6: octahedral, 90° (SF₆).
Intermolecular forces
- Instantaneous dipole–induced dipole (van der Waals/London) forces: in all molecules; increase with the number of electrons.
- Permanent dipole–dipole forces: between polar molecules.
- Hydrogen bonds: H bonded to N, O or F interacting with a lone pair on N, O or F. They explain water's high boiling point and ice being less dense than water.
Structure and properties
Giant ionic, giant covalent (diamond, graphite, SiO₂), giant metallic and simple molecular structures each have characteristic melting points and conductivity.
Exam tip: To predict a shape, count ALL electron pairs around the central atom, then name the shape from the bonding pairs only.
Check yourself
- Predict the shape and bond angle of PH₃.
- Why is CO₂ non-polar even though C=O bonds are polar?
- Explain why H₂O has a higher boiling point than H₂S.
Show answers
- Pyramidal, about 107° (3 bonding pairs, 1 lone pair).
- The molecule is linear and symmetrical, so the dipoles cancel.
- Water forms hydrogen bonds between molecules; H₂S has only weaker dipole–dipole and London forces.
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