---
title: "Define effective atomic number (EAN)"
subject: "Engineering Chemistry"
chapter: 6
key: "semester_2/engineering_chemistry:define-ean"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/define-ean
site: BCT Engineering
---

# Define effective atomic number (EAN)

Engineering Chemistry · Chapter 6: Coordination Complexes

- Asked in: 2069 Poush
- Video: [Effective Atomic Number - Co-Ordination Compounds - Chemistry Class 12](https://www.youtube.com/watch?v=v5zO3gZExfU) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/define-ean
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-6

## Question

Define the term effective atomic number (EAN) with an example.

## Short answer

EAN is the total electrons around the central metal: Z – oxidation state + 2×(number of donor electrons from ligands). Stable complexes match a noble gas; e.g., [Fe(CN)₆]⁴⁻ has EAN = 24+12 = 36 (krypton).

## Answer

The **effective atomic number (EAN)** of a central metal is the total number of electrons around it in a complex, i.e. its own electrons (after ionisation) plus two electrons from each ligand donor.

EAN = Z − (oxidation state) + 2 × (coordination number for monodentate ligands)

According to Sidgwick, stable complexes have an EAN equal to the next noble gas.

Example: [Fe(CN)₆]⁴⁻: Fe²⁺ has 24 e⁻; adding 6 × 2 = 12 gives EAN = 36 (krypton).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does EAN of a central metal represent?
   A. Atomic number of the metal
   B. Number of ligands
   C. Total electrons around the metal: its own (after ionisation) plus 2 from each ligand donor
   D. Oxidation state plus coordination number
   Answer: C. EAN counts the metal's remaining electrons plus those donated by ligands. (Key point: Meaning of EAN)

2. According to Sidgwick, when are complexes stable?
   A. When EAN equals the atomic number of the next noble gas
   B. When EAN equals the atomic number of the metal
   C. When EAN is zero
   D. When EAN is odd
   Answer: A. Stable complexes tend to attain a noble gas electron count. (Key point: Sidgwick rule)

3. How many electrons does Fe2+ (Z = 26) have?
   A. 26
   B. 28
   C. 23
   D. 24
   Answer: D. 26 - 2 = 24. (Key point: Fe2+ electrons)

4. What is EAN of Fe in [Fe(CN)6]4-?
   A. 30
   B. 36 (krypton)
   C. 34
   D. 54
   Answer: B. 24 + 6 x 2 = 36, the atomic number of krypton. (Key point: EAN of [Fe(CN)6]4-)

## Related in Engineering Chemistry

- [Effective atomic number and IUPAC names](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aean-iupac-2071) — Engineering Chemistry, chapter 6
- [IUPAC names and EAN of complexes](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aiupac-ean) — Engineering Chemistry, chapter 6

## Related in other subjects

- [nth Fibonacci term prime check](https://www.bctengineer.com/question.md?key=semester_1%2Fcomputer_programming%3Afibonacci-nth-prime) — Computer Programming, chapter 6