Periodicity is the repeating pattern of physical and chemical properties shown by elements when arranged in increasing atomic number. Periodic trends are predictable changes in these properties across a period or down a group.
Why Periodicity Occurs
Periodicity results from repeating patterns in electron configuration. Elements in the same group have the same number of valence electrons, giving them similar chemical properties.
Across a period, proton number increases while electrons are added to the same energy level. Greater nuclear charge strengthens attraction to valence electrons. Down a group, additional occupied energy levels increase atomic radius and electron shielding, weakening attraction to outer electrons.
| Trend | Across a period | Down a group |
|---|---|---|
| Atomic radius | Decreases as nuclear charge increases with similar shielding | Increases as additional energy levels are occupied |
| First ionization energy | Increases as the outer electron is more strongly attracted | Decreases as distance and shielding increase |
| Electronegativity | Generally increases | Generally decreases |
| Metallic character | Generally decreases | Generally increases |
Across a period, elements generally change from metallic to non-metallic character, while their oxides change from basic oxides, through amphoteric oxides, to acidic oxides. Down group 1, increasing atomic radius and shielding make electron loss easier, so metallic reactivity increases. Down group 17, attraction for an incoming electron weakens, so non-metallic reactivity decreases.
A common misconception is that every trend is perfectly smooth. Discontinuities can occur because of sublevels and electron pairing. Explaining first ionization energy discontinuities is additional higher level content; core periodicity is studied at SL and HL. This links electron configurations in Structure 1.3 to periodicity in Structure 3.1.
Exam Technique
For an explain question, state the trend and justify it using nuclear charge, occupied energy levels, shielding, and attraction for valence electrons. Do not write that a property changes “because of periodicity”; explain the particle-level cause.