Periodic trends explained by cause, not memorisation
Every trend comes from two competing effects: nuclear charge and shielding. Learn the cause and you never need the table of exceptions.
What this covers
- Atomic and ionic radius
- Ionisation energy
- Electronegativity
- Shielding and effective nuclear charge
- Group and period reactivity
Worked example
Problem
Why does ionisation energy increase across a period?
- 1What changesProtons increase across the period while electrons enter the same shell.
- 2ShieldingShielding stays roughly constant, so effective nuclear charge rises.
- 3ConsequenceOuter electrons are held more tightly and are harder to remove.
Answer: Higher effective nuclear charge with similar shielding means more energy is needed to remove an electron.
Mistakes that cost marks
- Explaining trends by 'more electrons' rather than effective nuclear charge.
- Ignoring the small dips at group 13 and 16.
- Comparing an atom's radius with an ion's radius as if they were equivalent.
Questions students ask
Do I need to memorise exceptions?
Only two patterns — the s/p sub-shell dip and the paired-electron repulsion dip — both follow from orbital structure.
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