Atomic & Ionic Radii General Trend

Periodic Trends: $$\begin{aligned} \text{Across a Period (Left to Right)} &\implies \text{Radii Decrease} \\ \text{Down a Group (Top to Bottom)} &\implies \text{Radii Increase} \end{aligned}$$

Concept: Radii contract across a period due to increasing effective nuclear charge drawing electrons closer, and expand down a group due to the addition of new principal electronic shells.

Notation:

• $$\text{Across period}$$ = Constant shell number with rising$$Z_{\text{eff}}$$;
• $$\text{Down group}$$ = Dominant effect of increasing principal quantum number ($$n$$).