Guides
Periodic Trends Summary — Atomic Radius, Electronegativity, Metallic Character
The periodic law states that element properties repeat periodically as atomic number increases. Learn a few core trends and you can infer most elements' relative properties without memorizing them. The trends come from the tug-of-war between nuclear charge and the number of electron shells.
Four core trends
| Property | Across a period (L→R) | Down a group (top→bottom) |
|---|---|---|
| Atomic radius | decreases | increases |
| Electronegativity | increases | decreases |
| Metallic character | decreases | increases |
| Nonmetallic char. / ionization energy | increases | decreases |
Why
- Across a period: the number of shells stays the same while nuclear charge rises, pulling outer electrons in harder → smaller radius, higher electronegativity and nonmetallic character.
- Down a group: more shells mean the outer electrons are farther out and more shielded → larger radius, easier to lose electrons, stronger metallic character.
How to use it
Use these to rank unknown elements: within a period, F is more electronegative than O; within a group, K is more reactive than Na. For values and visual comparisons see the visualization center, or read the electronegativity and atomic radius guides.