Core Electrons Versus Valence Electrons
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Valence electrons occupy an atom's outermost shell, while core electrons fill the complete inner shells beneath, matching a noble gas's configuration.
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Sodium's configuration, [Ne]3s¹, shows its ten core electrons abbreviated as [Ne], leaving one valence electron in the 3s orbital.
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Chemical reactivity is governed chiefly by valence electrons — core electrons are shielded from most chemical interactions by the shells outside them.