Chemistry unit guide
Models of bonding and structure
Ionic, covalent and metallic bonding, Lewis structures, VSEPR shapes, polarity and intermolecular forces.
Structure 2 is where the course starts rewarding precise language. Nearly every mark here is earned by naming the right force and explaining a property in terms of it, and the difference between a 4 and a 7 is usually whether a student says "intermolecular forces" or names the specific force. Paper 1 tests shapes and polarity heavily because both are quick to examine with multiple choice; Paper 2 asks you to explain physical properties from structure.
Topics in this unit
What gets examined
- Drawing Lewis structures including expanded octets, and deducing shape and bond angle from electron domains.
- Distinguishing electron domain geometry from molecular geometry when lone pairs are present.
- Predicting whether a molecule is polar from its shape and bond dipoles, not just from its bonds.
- Ranking boiling points by identifying the strongest intermolecular force present in each substance.
- Explaining conductivity, melting point and solubility in terms of structure and bonding.
- The metallic bonding model and how it explains malleability and conductivity.
How to revise it
Name the force, never the category
"Hydrogen bonding", "dipole-dipole", "London dispersion". A mark scheme that wants hydrogen bonding will not accept "intermolecular forces", and this single habit recovers marks across the whole paper.
Count domains, then subtract lone pairs
Total electron domains give the geometry; lone pairs then compress the angles and change the shape name. Practising this as two separate steps stops the classic error of calling a bent molecule linear.
Learn the bond angle numbers exactly
109.5°, 107°, 104.5°, 120°, 180°. These are recalled, not derived, and questions ask for them directly.
Explain properties in a chain
Structure, then force, then energy needed to overcome it, then the property. Mark schemes award along that chain, so writing it out earns marks even where the conclusion is obvious.
Where marks get lost
- Saying a molecule is polar because it contains polar bonds, ignoring that symmetry can cancel them.
- Describing intermolecular forces as "bonds broken" when a substance melts - covalent bonds within molecules stay intact.
- Drawing Lewis structures without the lone pairs the question needs to justify the shape.
- Confusing electron domain geometry with molecular shape in the answer.
- Attributing high melting point to hydrogen bonding in a substance that is actually a giant covalent structure.
Practise this unit
Reading about a unit only goes so far. Work through real IB-style questions on it, with mark schemes.