effective atomic number rule

chemistry

Also known as: EAN rule

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Last updated

June 26, 2026

Quick Summary

The effective atomic number (EAN) rule, proposed by Nevil V. Sidgwick, is a principle in chemistry that describes the number of electrons surrounding a metal atom in a complex. This number includes the metal atom's own electrons plus those contributed by surrounding ligands. The EAN rule suggests that metal atoms in complexes tend to achieve an electron count equal to that of the nearest noble gas in the periodic table. This is often referred to as the "18-electron rule" when considering only valence electrons. This rule is particularly useful for predicting the number of ligands in metal carbonyls and other organometallic compounds.1

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effective atomic number
…observation, since known as the EAN rule, that in a number of metal complexes the metal atom tends to surround itself with sufficient ligands that the resulting effective atomic number is numerically equal to the atomic number of the noble-gas element found in the same period in which the metal…
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…observation, since known as the EAN rule, that in a number of metal complexes the metal atom tends to surround itself with sufficient ligands that the resulting effective atomic number is numerically equal to the atomic number of the noble-gas element found in the same period in which the metal…
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metal carbonyl
Structure and bonding
…18-electron rule, also called the effective atomic number (EAN) rule, which states that stable complexes often have 18 valence electrons, mirroring the electron configuration of noble gases.
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…18-electron rule, also called the effective atomic number (EAN) rule, which states that stable complexes often have 18 valence electrons, mirroring the electron configuration of noble gases.
Read More