Periodic Trends Explained
Chemistry Tools | May 20, 2026 | 9 min read
Atomic radius, ionization energy, electronegativity, and how to reason across the periodic table.
| Trend | Across a period | Down a group |
| Atomic radius | Decreases | Increases |
| Ionization energy | Increases | Decreases |
| Electronegativity | Increases | Decreases |
| Metallic character | Decreases | Increases |
The Main Idea
Periodic trends come from attraction between the nucleus and electrons. Across a period, protons increase while shielding does not increase much, so valence electrons feel a stronger effective nuclear charge.
Down a group, atoms gain electron shells. Extra distance and shielding make the outer electrons less tightly held, even though nuclear charge is larger.
Atomic Radius
Atomic radius decreases from left to right because electrons are pulled closer by increasing effective nuclear charge. Radius increases down a group because new shells place valence electrons farther from the nucleus.
This explains why fluorine is small and reactive, while cesium is large and loses electrons easily.
Ionization and Electronegativity
Ionization energy is the energy required to remove an electron. It generally increases across a period and decreases down a group. Electronegativity measures attraction for bonding electrons and follows a similar direction.
Exceptions exist because filled and half-filled subshells have special stability, but the broad pattern is enough for most first-pass reasoning.
Useful Site Tools
Use the Interactive Periodic Table for element data and the Electronegativity Chart page for bond polarity comparisons.