Chemistry unit guide
Classification of matter
Periodic trends, functional groups, isomerism and the three spectroscopic techniques.
Structure 3 splits into trends you explain and spectra you read. The periodic trends half rewards one habit above all: every explanation comes back to nuclear charge, shielding and atomic radius, and an answer that names all three earns the marks whatever the specific trend. The spectroscopy half is pure pattern recognition, and it is the most improvable content in the course - the number of distinct things you can be asked is small, and an hour with the data booklet is worth more than a week of reading.
Topics in this unit
What gets examined
- Explaining trends in atomic radius, ionisation energy and electronegativity across a period and down a group.
- Identifying functional groups from a structure and naming compounds using IUPAC rules.
- Distinguishing structural isomers from stereoisomers, and cis-trans from optical isomerism.
- Deducing the molecular ion peak and fragment losses from a mass spectrum.
- Matching infrared absorptions to bonds using section 20 of the data booklet.
- Counting environments in a proton NMR spectrum and interpreting integration and splitting at HL.
How to revise it
Anchor every trend to the same three factors
Nuclear charge, shielding, atomic radius. Whatever the trend, name all three and the direction of each. Mark schemes are built around them, and this turns a whole section of the course into one memorised structure.
Learn the fragment losses, not the spectra
Loss of 15 is a methyl group, 17 a hydroxyl, 29 an ethyl or a CHO, 45 a COOH. Four numbers cover most of what mass spectrometry questions ask.
Use the data booklet during revision, not just in the exam
IR and NMR questions are open-book by design. Practising with section 20 and 27 in front of you builds the lookup speed the exam actually tests.
Count NMR environments by symmetry
Draw the structure, then mark chemically equivalent hydrogens. The number of distinct environments is the number of peaks, and getting that right is most of the mark.
Where marks get lost
- Explaining a trend with nuclear charge alone, without mentioning shielding or radius.
- Confusing electron affinity with electronegativity.
- Calling geometric isomers optical isomers - optical isomerism needs a chiral centre.
- Reading an IR absorption as a bond without checking whether it is in the fingerprint region.
- Counting hydrogens rather than hydrogen environments when predicting NMR peaks.
Practise this unit
Reading about a unit only goes so far. Work through real IB-style questions on it, with mark schemes.