Inorganic chemistry has a reputation for being the part you simply have to memorise. Some of it is. Much more of it than students expect follows from a handful of trends — and the exceptions are memorable precisely because they are exceptions.
The four that do most of the work
- Atomic radius — decreases across a period, increases down a group.
- Ionisation enthalpy — increases across, decreases down.
- Electronegativity — increases across, decreases down.
- Metallic character — decreases across, increases down.
Nearly every "arrange in increasing order" question is one of these, and recognising which one takes a moment once the four are genuinely automatic.
Why the trends exist is worth one paragraph
Across a period, nuclear charge rises while the shell stays the same, so electrons are held closer and more tightly. Down a group, a new shell is added and shielding increases, so the outer electron is further out and easier to remove.
Two sentences. Knowing them means you can reconstruct any of the four trends if it slips, rather than having four unrelated facts to hold.
The exceptions are the questions
A question that follows the trend is easy and therefore less useful to an examiner. The ones that appear are usually where the trend breaks:
- Ionisation enthalpy anomalies at half-filled and fully-filled configurations — stability interrupting the pattern.
- The lanthanoid contraction and its effect further down.
- Electron gain enthalpy where the second-row element is not the extreme, because of its small size.
Learn these as "the trend says X, but here Y, because Z". The reason is what makes it stick and what lets you handle a variant.
Colour, solubility and tests
Some of it genuinely is memory. Keep it small and organised:
- Characteristic flame colours.
- Precipitate colours in common tests.
- Which compounds are soluble and which are not.
One page, revisited weekly, is better than these facts scattered across chapters where you will never find them again.