---
title: "Define co-ordination number with example"
subject: "Engineering Chemistry"
chapter: 6
key: "semester_2/engineering_chemistry:define-coordination-number"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/define-coordination-number
site: BCT Engineering
---

# Define co-ordination number with example

Engineering Chemistry · Chapter 6: Coordination Complexes

- Asked in: 2069 Poush
- Video: [How to find coordination number? | How to find oxidation state? | Coordination Compounds](https://www.youtube.com/watch?v=FtCHGI1soy8) — Najam Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/define-coordination-number
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-6

## Question

Define the term co-ordination number with an example.

## Short answer

Coordination number is the number of ligand donor atoms directly bonded to the central metal ion. In [Co(NH₃)₆]³⁺ it is 6; in [Cu(en)₂]²⁺ it is 4.

## Answer

The **coordination number** of a central metal ion is the total number of ligand donor atoms directly bonded to it by coordinate bonds (the number of coordinate bonds formed). It equals Werner's secondary valency.

Examples:
- [Co(NH₃)₆]³⁺: coordination number 6.
- [Ni(CN)₄]²⁻: coordination number 4.
- [Cu(en)₂]²⁺: coordination number 4 (each en is bidentate, two donor atoms).

## Check yourself

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

1. What is the coordination number of a central metal ion?
   A. The charge on the metal ion
   B. The number of ligand donor atoms directly bonded to it by coordinate bonds
   C. The number of ligand molecules whatever their denticity
   D. The number of counter ions
   Answer: B. It is the total number of coordinate bonds formed to the metal. (Key point: Definition of CN)

2. What is the coordination number in [Cu(en)2]2+?
   A. 2
   B. 6
   C. 3
   D. 4
   Answer: D. Each en is bidentate with two donor atoms, so 2 x 2 = 4. (Key point: CN of [Cu(en)2]2+)

3. Coordination number equals which Werner valency?
   A. Secondary valency
   B. Primary valency
   C. Oxidation state
   D. Ionic valency
   Answer: A. Secondary (non-ionisable) valency equals the coordination number. (Key point: Link to Werner)

4. What is the coordination number of Co in [Co(NH3)6]3+?
   A. 3
   B. 9
   C. 6
   D. 2
   Answer: C. Six NH3 each donate one lone pair, so six coordinate bonds. (Key point: CN of [Co(NH3)6]3+)

## Related in Engineering Chemistry

- [VBT formation of [Ni(CO)₄] and [Fe(CN)₆]⁴⁻](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Avbt-nico4-fecn6-4) — Engineering Chemistry, chapter 6
- [Define effective atomic number (EAN)](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Adefine-ean) — Engineering Chemistry, chapter 6

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