Overview
Concise notes on ionic radii: size trends across periods and groups, effects of charge, and comparisons using examples and values.
Periodic Trends of Ionic Radii
- Ionic radii increase down a group; ions become larger with more energy levels.
- Ionic radii decrease across a period to the right; higher effective nuclear charge contracts ions.
- Crossing from cations to anions shows a jump; anions are much larger than cations.
Quantitative Examples of Trends
- Li+ ≈ 60 pm; Na+ ≈ 95 pm; K+ ≈ 133 pm; size increases down group.
- Mg2+ ≈ 65 pm; Al3+ ≈ 50 pm; size decreases to the right with higher charge.
- S2− ≈ 184 pm; Cl− ≈ 181 pm; anions are larger than nearby cations.
Table of Selected Ionic Radii
| Ion/Atom | Charge | Electrons | Energy Levels (qual.) | Ionic/Atomic Radius (pm) |
|---|
| Li | 0 | 3 | 2 shells | 186 |
| Li+ | +1 | 2 | 1 shell | 60 |
| Na | 0 | 11 | 3 shells | 186 |
| Na+ | +1 | 10 | 2 shells | 95 |
| K+ | +1 | 18 | 3 shells | 133 |
| Be2+ | +2 | 2 | 1 shell | 31 |
| Mg2+ | +2 | 10 | 2 shells | 65 |
| Al3+ | +3 | 10 | 2 shells | 50 |
| Ca2+ | +2 | 18 | 4 shells (vs Be/Mg) | 99 |
| Ga3+ | +3 | 28 (neutral), ion has fewer | More contracted | 62 |
| S2− | −2 | 18 | 3 shells | 184 |
| Cl | 0 | 17 | 3 shells | 99 |
| Cl− | −1 | 18 | 3 shells (expanded) | 181 |
| Br− | −1 | 36 | 4 shells | 195 |
Charge Effects on Ionic Size
- More positive charge yields smaller ions; electron loss reduces radius.
- More negative charge yields larger ions; electron gain increases repulsion and radius.
- Effective nuclear charge increase contracts electron cloud; decreases ionic radius.
Comparisons and Reasoning Examples
- Mg2+ larger than Be2+; Mg2+ has two energy levels vs one for Be2+.
- Ga3+ smaller than Ca2+; higher charge and position to the right reduce size.
- Br− larger than K+; anion has more electrons and an extra energy level.
- Cation vs parent atom: cation is smaller; loss of a shell reduces size.
- Anion vs parent atom: anion is larger; added electron expands electron cloud.
Isoelectronic Series Ranking
- Isoelectronic means same electron configuration and electron count.
- Example set: O2−, F−, Na+, Mg2+; all have 10 electrons (1s2 2s2 2p6).
- Decreasing ionic radii order: O2− > F− > Na+ > Mg2+.
Key Terms & Definitions
- Ionic radius: Measure of an ion’s size in picometers (pm).
- Cation: Positively charged ion; formed by electron loss; typically smaller.
- Anion: Negatively charged ion; formed by electron gain; typically larger.
- Effective nuclear charge: Net positive charge felt by electrons; higher values shrink radii.
- Isoelectronic: Species with the same number of electrons and configuration.
Action Items / Next Steps
- Practice ranking ionic sizes across periods, down groups, and across charges.
- Memorize typical radii trends and key example values for common ions.
- Analyze isoelectronic series by charge magnitude to predict relative sizes.