Radii decrease left-to-right across a period and increase top-to-bottom in a group; example orders: Group 1 (largest to smallest): Fr > Cs > Rb > K > Na > Li; Period 3 (largest to smallest): Na > Mg > Al > Si > P > S > Cl > Ar.
Explanation
Atomic and ionic radii are governed by effective nuclear charge and electron shielding. Across a period, the nucleus pulls electrons in more strongly with little extra shielding, so the atomic radius becomes smaller from left to right. Down a group, each successive element adds a new electron shell, increasing the overall size. Ionic radii also follow trends: cations (positive ions) are smaller than their neutral atoms, while anions (negative ions) are larger due to electron-electron repulsion and added electrons. Therefore, elements can be ranked by decreasing radii using these general rules and example sequences.
Key Points
- 1, Radii generally decrease across a period from left to right and increase down a group.
- 2, Group 1 example (largest to smallest): Fr > Cs > Rb > K > Na > Li.
- 3, Period 3 example (largest to smallest): Na > Mg > Al > Si > P > S > Cl > Ar. Ionic radii follow cations < neutral atoms < anions.