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CHEM 121 Studioby Learn4Less · UBC CHEM 121

CHEM 121 glossary

Effective nuclear charge

The net positive charge a valence electron actually feels, the nuclear charge ZZ reduced by shielding. In this course's simple model Zeff≈Z−(core electrons)Z_{\text{eff}} \approx Z - (\text{core electrons}): it rises by about one per element across a period and stays roughly constant down a group.

Bar chart of values in Zeff (simple model)Na: 1 Zeff (simple model); Mg: 2 Zeff (simple model); Al: 3 Zeff (simple model); Si: 4 Zeff (simple model); P: 5 Zeff (simple model); S: 6 Zeff (simple model); Cl: 7 Zeff (simple model); Ar: 8 Zeff (simple model)02468Zeff (simple model)1Na2Mg3Al4Si5P6S7Cl8Ar

Period 3: every element has 10 core electrons, so ZeffZ_{\text{eff}} climbs by one each step.

Example

Fluorine: Z=9Z = 9, 2 core electrons, Zeff≈7Z_{\text{eff}} \approx 7. Lithium: 3−2=13 - 2 = 1. Fluorine's valence electrons are held far more tightly, so F is smaller and harder to ionize.

See alsoShieldingCore electronAtomic radiusIonization energy