Periodic Trends Across Periods and Down Groups
- Atomic Radius: Decreases across a period (increasing effective nuclear charge $Z_{\text{eff}}$); increases down a group (new shells added).
- Ionization Enthalpy: Increases across a period; decreases down a group.
- Electron Gain Enthalpy: Becomes more negative across a period; less negative down a group (chlorine is more negative than fluorine due to compact $2p$ repulsion in fluorine).
