---
title: "Effective atomic number and IUPAC names"
subject: "Engineering Chemistry"
chapter: 6
key: "semester_2/engineering_chemistry:ean-iupac-2071"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/ean-iupac-2071
site: BCT Engineering
---

# Effective atomic number and IUPAC names

Engineering Chemistry · Chapter 6: Coordination Complexes

- Asked in: 2071 Bhadra, 1+4 marks
- 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/ean-iupac-2071
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-6

## Question

What do you mean by effective atomic number? Give IUPAC name and calculate the effective atomic number of the following complexes.
a) [Fe(CN)₆]³⁻
b) [Ag(NH₃)₄]⁺
c) [Ni(CN)₄]²⁻
d) [Cr(H₂O)₆]³⁺

## Short answer

Effective atomic number (EAN) = (Z – oxidation state) + 2×coordination number. IUPAC: [Fe(CN)6]3- hexacyanoferrate(III) ion, EAN=35; [Ag(NH3)4]+ tetraamminesilver(I) ion, EAN=54; [Ni(CN)4]2- tetracyanonickelate(II) ion, EAN=34; [Cr(H2O)6]3+ hexaaquachromium(III) ion, EAN=33.

## Answer

#### Effective atomic number (EAN)
The EAN is the total number of electrons around the central metal atom/ion in a complex, counting the electrons gained by coordination. EAN = (Z − oxidation state) + 2 × (number of ligands). Stable complexes often have EAN equal to the next noble gas (Sidgwick's rule).

| Complex | IUPAC name | Z | Ox. state | Calculation | EAN |
|---|---|---|---|---|---|
| [Fe(CN)₆]³⁻ | hexacyanoferrate(III) ion | 26 | +3 | 26 − 3 + 12 | 35 |
| [Ag(NH₃)₄]⁺ | tetraamminesilver(I) ion | 47 | +1 | 47 − 1 + 8 | 54 |
| [Ni(CN)₄]²⁻ | tetracyanonickelate(II) ion | 28 | +2 | 28 − 2 + 8 | 34 |
| [Cr(H₂O)₆]³⁺ | hexaaquachromium(III) ion | 24 | +3 | 24 − 3 + 12 | 33 |

**Answer:** EAN = 35, 54, 34 and 33 respectively. (Kr = 36 and Xe = 54, so only the silver complex reaches a noble gas value.)

## Check yourself

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

1. What is meant by the effective atomic number (EAN)?
   A. Z of the metal
   B. number of ligands
   C. oxidation state
   D. total electrons around metal in complex
   Answer: D. The correct option is: total electrons around metal in complex. This follows from the answer on ean. (Key point: EAN)

2. What is the EAN of Fe in [Fe(CN)6]3-?
   A. 35
   B. 36
   C. 33
   D. 34
   Answer: A. The correct option is: 35. This follows from the answer on fe. (Key point: Fe)

3. What is the EAN of Ag in [Ag(NH3)4]+?
   A. 47
   B. 54
   C. 46
   D. 36
   Answer: B. The correct option is: 54. This follows from the answer on ag. (Key point: Ag)

4. Which of these complexes reaches a noble gas EAN?
   A. iron
   B. nickel
   C. silver complex
   D. chromium
   Answer: C. The correct option is: silver complex. This follows from the answer on noble. (Key point: Noble)

## Related in Engineering Chemistry

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