Electron structure explains the pattern.
Electrons fill energy levels and subshells, with valence electrons strongly influencing bonding. Across a period, increasing nuclear charge generally strengthens attraction to valence electrons in the same principal shell. Atomic radius tends to decrease and first ionisation energy tends to increase, with subshell and pairing exceptions.
Down a group, occupied shells are added. Greater distance and shielding generally increase atomic radius and reduce first ionisation energy. Cations are often smaller than their parent atoms; anions are often larger because additional electron repulsion expands the electron distribution.
A simplified attraction model
Explore the relationship
A qualitative inverse-square analogy. Real atomic radii and ionisation energies require quantum structure and shielding; this is not a quantitative atom prediction.
Tap or focus a plotted point to read its values.
View plotted data as a table
| Series | Electron distance / relative units | Attraction magnitude / relative units |
|---|---|---|
| Attraction magnitude / relative units | 1 | 2 |
| Attraction magnitude / relative units | 1.05 | 1.81406 |
| Attraction magnitude / relative units | 1.1 | 1.65289 |
| Attraction magnitude / relative units | 1.15 | 1.51229 |
| Attraction magnitude / relative units | 1.2 | 1.38889 |
| Attraction magnitude / relative units | 1.25 | 1.28 |
| Attraction magnitude / relative units | 1.3 | 1.18343 |
| Attraction magnitude / relative units | 1.35 | 1.09739 |
| Attraction magnitude / relative units | 1.4 | 1.02041 |
| Attraction magnitude / relative units | 1.45 | 0.951249 |
| Attraction magnitude / relative units | 1.5 | 0.888889 |
| Attraction magnitude / relative units | 1.55 | 0.832466 |
| Attraction magnitude / relative units | 1.6 | 0.78125 |
| Attraction magnitude / relative units | 1.65 | 0.734619 |
| Attraction magnitude / relative units | 1.7 | 0.692042 |
| Attraction magnitude / relative units | 1.75 | 0.653061 |
| Attraction magnitude / relative units | 1.8 | 0.617284 |
| Attraction magnitude / relative units | 1.85 | 0.584368 |
| Attraction magnitude / relative units | 1.9 | 0.554017 |
| Attraction magnitude / relative units | 1.95 | 0.52597 |
| Attraction magnitude / relative units | 2 | 0.5 |
| Attraction magnitude / relative units | 2.05 | 0.475907 |
| Attraction magnitude / relative units | 2.1 | 0.453515 |
| Attraction magnitude / relative units | 2.15 | 0.432666 |
| Attraction magnitude / relative units | 2.2 | 0.413223 |
| Attraction magnitude / relative units | 2.25 | 0.395062 |
| Attraction magnitude / relative units | 2.3 | 0.378072 |
| Attraction magnitude / relative units | 2.35 | 0.362155 |
| Attraction magnitude / relative units | 2.4 | 0.347222 |
| Attraction magnitude / relative units | 2.45 | 0.333195 |
| Attraction magnitude / relative units | 2.5 | 0.32 |
| Attraction magnitude / relative units | 2.55 | 0.307574 |
| Attraction magnitude / relative units | 2.6 | 0.295858 |
| Attraction magnitude / relative units | 2.65 | 0.284799 |
| Attraction magnitude / relative units | 2.7 | 0.274348 |
| Attraction magnitude / relative units | 2.75 | 0.264463 |
| Attraction magnitude / relative units | 2.8 | 0.255102 |
| Attraction magnitude / relative units | 2.85 | 0.24623 |
| Attraction magnitude / relative units | 2.9 | 0.237812 |
| Attraction magnitude / relative units | 2.95 | 0.229819 |
| Attraction magnitude / relative units | 3 | 0.222222 |
| Attraction magnitude / relative units | 3.05 | 0.214996 |
| Attraction magnitude / relative units | 3.1 | 0.208117 |
| Attraction magnitude / relative units | 3.15 | 0.201562 |
| Attraction magnitude / relative units | 3.2 | 0.195312 |
| Attraction magnitude / relative units | 3.25 | 0.189349 |
| Attraction magnitude / relative units | 3.3 | 0.183655 |
| Attraction magnitude / relative units | 3.35 | 0.178213 |
| Attraction magnitude / relative units | 3.4 | 0.17301 |
| Attraction magnitude / relative units | 3.45 | 0.168032 |
| Attraction magnitude / relative units | 3.5 | 0.163265 |
| Attraction magnitude / relative units | 3.55 | 0.158699 |
| Attraction magnitude / relative units | 3.6 | 0.154321 |
| Attraction magnitude / relative units | 3.65 | 0.150122 |
| Attraction magnitude / relative units | 3.7 | 0.146092 |
| Attraction magnitude / relative units | 3.75 | 0.142222 |
| Attraction magnitude / relative units | 3.8 | 0.138504 |
| Attraction magnitude / relative units | 3.85 | 0.13493 |
| Attraction magnitude / relative units | 3.9 | 0.131492 |
| Attraction magnitude / relative units | 3.95 | 0.128185 |
| Attraction magnitude / relative units | 4 | 0.125 |
Explore: Compare attraction at greater distance and larger effective nuclear charge. Note the model’s limits.
Explain why Na⁺ is smaller than Na.
- Na loses its 3s valence electron.
- The remaining electrons occupy lower principal shells and are held more tightly.
Assumed knowledge
Proton number and electron shells.
Learning checkpoints & source
YOUR LEARNING CHECKPOINT- Relate electron configuration to periodic position.
- Explain radius and first-ionisation-energy trends.