3.1.3—Periodicity
- Syllabus
- First assessment 2025
- Objective
- 3.1.3
- Level
- HL
Periodic Trends
| Quantity | Across a period | Down a group |
|---|---|---|
| Atomic/ionic radius | Generally decreases | Generally increases |
| First IE | Generally increases | Generally decreases |
| Electronegativity | Generally increases | Generally decreases |
| Electron affinity | Interpret with the stated convention and attraction evidence | Interpret with shell and shielding evidence |
Explain a trend with effective nuclear charge, shielding, shell, distance, and attraction; a direction alone is not a complete explanation.
Across period 3, nuclear charge rises while added electrons enter the same main shell, so effective attraction generally increases, radius falls and first ionization energy rises. For ions, compare electron count and charge as well as position; an isoelectronic species with more protons is smaller.
Electron affinity needs a sign check. Under the enthalpy-change convention, a more favourable first electron gain is more negative: it generally becomes more negative across a period as nuclear attraction increases, and less negative down a group as distance and shielding increase. Sublevel energy and electron repulsion cause exceptions, so compare the stated data rather than forcing every element into a smooth trend.
Explaining Periodic Trends
Assessment in practice
Representative question
Question 1
[Maximum number: 2]
Explain why the first ionization energy decreases as you descend group 15 from nitrogen to bismuth.
«electron removed from» higher orbital/shell/energy level / further away from the nucleus.
more shielded/lower attractive force «between the nucleus and outer electron».
Marking guidance:
Do not accept increase in atomic radius on its own for M1
The Periodic Table Summary
Retrieve the route: locate an element from configuration, explain periodic and group trends, write oxide/reaction and oxidation-state answers, then connect incomplete d-sublevels to transition properties, ion configurations, and colours.
Check that every trend explanation names its particle-level cause, every equation is balanced, every oxidation state is a formal charge convention, and every transition colour uses absorbed/observed complementarity.