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CHEM 121 Studioby Learn4Less · UBC CHEM 121

CHEM 121 glossary

Core electron

An electron in a filled inner shell, below the valence shell; together they make up the noble-gas core. Core electrons do not take part in bonding, but they shield the valence electrons from the nucleus.

Shielding and effective nuclear charge in sodiumSodium's nucleus has charge +11. Ten core electrons sit between the nucleus and the single 3s valence electron and cancel most of that charge, so the valence electron feels an effective nuclear charge of about +1 (Z − core = 11 − 10).+113s e⁻Na: Z = 1110 core e⁻ shieldZeff ≈ 11 − 10≈ +1shaded core electrons cancel most of the +11

Example

Silicon, [Ne] 3s2 3p2[\mathrm{Ne}]\,3s^2\,3p^2, has 10 core electrons (1s2 2s2 2p61s^2\,2s^2\,2p^6) and 4 valence electrons.

See alsoValence electronShieldingNoble-gas configurationEffective nuclear charge