---
title: "Why transition elements are coloured and paramagnetic"
subject: "Engineering Chemistry"
chapter: 5
key: "semester_2/engineering_chemistry:tm-colour-paramagnetic"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/tm-colour-paramagnetic
site: BCT Engineering
---

# Why transition elements are coloured and paramagnetic

Engineering Chemistry · Chapter 5: 3-d Transition Elements and their Applications

- Asked in: 2070 Bhadra, 4 marks
- Video: [Why do Transition Metals Form Coloured Compounds?](https://www.youtube.com/watch?v=mxBD-1aTB6E) — Chemistry Student
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/tm-colour-paramagnetic
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-5

## Question

Give reasons why transition elements are coloured and paramagnetic.

## Short answer

Colour arises from an incomplete d subshell; ligand field splitting allows d–d transitions that absorb visible light, transmitting the complementary colour. Paramagnetism is due to unpaired d electrons, each acting as a magnet; magnetic moment μ = √(n(n+2)) BM where n is the number of unpaired electrons.

## Answer

#### Coloured compounds
Transition metal ions have an incomplete d subshell. In the field of ligands the d orbitals split into two energy levels (t₂g and e_g). The energy gap lies in the visible region, so an electron can absorb a part of visible light and jump to the higher level (d–d transition). The remaining light is transmitted and appears as the complementary colour. Example: Cu²⁺ is blue, Fe³⁺ is yellow-brown, Mn²⁺ is pale pink. Ions with d⁰ or d¹⁰ (Sc³⁺, Zn²⁺) are colourless.

#### Paramagnetism
Paramagnetism is due to unpaired electrons, each acting as a small magnet. Transition metal ions have partly filled d orbitals that contain unpaired electrons (Fe³⁺ has 5, Cu²⁺ has 1), so they are attracted into a magnetic field. The magnetic moment is given by the spin-only formula μ = √(n(n+2)) BM, where *n* is the number of unpaired electrons. Mn²⁺ (n = 5) gives 5.92 BM.

## Check yourself

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

1. Why are transition metal ions coloured?
   A. They emit visible light from excited s electrons
   B. They contain filled s orbitals only
   C. They are always ionic with large radius
   D. d orbitals split in the ligand field and electrons absorb visible light in d-d transitions
   Answer: D. The energy gap lies in the visible region, so part of the light is absorbed. (Key point: Why coloured)

2. Which ions are colourless?
   A. d0 or d10 ions such as Sc3+ and Zn2+
   B. d5 ions such as Mn2+
   C. d9 ions like Cu2+
   D. d3 ions
   Answer: A. They have no d-d transition possible. (Key point: Colourless ions)

3. What causes paramagnetism?
   A. Paired electrons
   B. Unpaired electrons
   C. Filled s orbitals
   D. High atomic mass
   Answer: B. Each unpaired electron acts as a tiny magnet and is attracted into a field. (Key point: Cause of paramagnetism)

4. Mn2+ has 5 unpaired electrons. What is its spin-only moment?
   A. 2.83 BM
   B. 1.73 BM
   C. 5.92 BM
   D. 3.87 BM
   Answer: C. mu = sqrt(n(n+2)) = sqrt(35) = 5.92 BM. (Key point: Spin-only formula)

## Related in Engineering Chemistry

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