# d-block elements and transition metals

> IB Chemistry HL · IB Chemistry HL 2025+
> Source: https://www.owlsprep.com/study/ib-chemistry-hl-u3-d-block-elements-and-transition/

This module clarifies the key difference between d-block elements and transition metals, covers their core characteristic properties assessed in IB HL Chemistry, and explains common exam questions on this foundational inorganic topic.

**Prerequisites:** [Electron configurations and subshell structure](https://www.owlsprep.com/study/ib-chem-hl-u2-electron-configuration/); [Periodic table structure and periodic trends](https://www.owlsprep.com/study/ib-chem-hl-u3-periodic-trends/)

## Learning objectives

- Distinguish between d-block elements and transition metals
- State and explain characteristic properties of transition metals
- Explain the origin of colour in transition metal ions
- Interpret oxidation states of transition metals in compounds

## Definitions: d-block vs Transition Metals

**d-block element** — Elements located between groups 3 and 12 of the periodic table, with their highest energy electron occupying a d orbital. Valence electrons occupy both the outermost s orbital and inner d orbitals.

*Example:* Scandium (Z=21) to Zinc (Z=30) in period 4 are all d-block elements

**Transition metal** — A d-block element that forms at least one stable ion with a partially filled d subshell. Excludes d-block elements where all stable ions have full or empty d subshells.

*Example:* Iron is a transition metal; scandium and zinc are not

**Worked example:** Determine which of Sc (Z=21), Fe (Z=26), Zn (Z=30) are transition metals. Explain your answer.

1. Write neutral atom electron configurations (4s fills before 3d):
2. $$Sc: [Ar] 3d^1 4s^2 \\ Fe: [Ar] 3d^6 4s^2 \\ Zn: [Ar] 3d^{10} 4s^2$$
3. Write configurations of the most stable ions: 4s electrons are lost first:
4. $$Sc^{3+}: [Ar] 3d^0 \\ Fe^{2+}: [Ar] 3d^6, Fe^{3+}: [Ar] 3d^5 \\ Zn^{2+}: [Ar] 3d^{10}$$
5. Check for any stable ion with a partially filled d subshell: Sc only forms $Sc^{3+}$ (empty d), Zn only forms $Zn^{2+}$ (full d). Fe has multiple stable ions with partially filled d subshells.
6. Conclusion: Only Fe is a transition metal.

> **Exam tip:** Always check the d configuration of the stable ion, not the neutral atom, when classifying transition metals.

## Characteristic Properties: Variable Oxidation States

Transition metals have several unique properties that set them apart from main group metals, all arising from the partially filled, low-energy d subshell. The most foundational is variable oxidation states.

**Variable oxidation states** — Ability to form stable compounds with different oxidation numbers, caused by the very similar energy levels of the outermost s and inner d orbitals, allowing different numbers of electrons to be lost or shared.

*Example:* Manganese has stable oxidation states from +2 to +7

**Worked example:** Calculate the oxidation state of Fe in $[Fe(CN)_6]^{3-}$ and Mn in $KMnO_4$.

1. For $[Fe(CN)_6]^{3-}$: each $CN^-$ ligand has a charge of -1. Let x = oxidation state of Fe:
2. $$x + (6 \times -1) = -3 \implies x = +3$$
3. For $KMnO_4$: K = +1, each O = -2, overall compound is neutral:
4. $$+1 + x + (4 \times -2) = 0 \implies x = +7$$
5. Answer: Fe = +3, Mn = +7

- Formation of coloured ions
- Complex ion formation
- Catalytic activity
- Paramagnetism from unpaired d electrons

## Origin of Colour in Transition Metal Ions

When a transition metal ion is surrounded by ligands (negative ions or neutral molecules with a lone pair), the five degenerate (equal energy) d orbitals split into two groups with different energy levels. This is called d orbital splitting.

> **info**
>
> When visible light passes through a solution of the ion, photons of a specific wavelength are absorbed to excite an electron from the lower energy d orbital to the higher energy d orbital (a d-d transition). The colour we observe is the complementary colour of the absorbed light that is transmitted.

**Worked example:** Explain why aqueous $CuSO_4$ is blue, while aqueous $ZnSO_4$ is colourless.

1. Write the electron configuration of each metal ion:
2. $$Cu^{2+}: [Ar] 3d^9 \\ Zn^{2+}: [Ar] 3d^{10}$$
3. Water ligands split the 3d orbitals of $Cu^{2+}$. $Cu^{2+}$ has a partially filled d subshell, so d-d transitions can occur. Red light is absorbed from visible light, and the complementary blue light is transmitted, giving the solution a blue colour.
4. $Zn^{2+}$ has a fully occupied d subshell. There are no empty higher energy d orbitals for an electron to be promoted into, so d-d transitions cannot occur. No visible light is absorbed, so the solution is colourless.

> **Exam tip:** You must mention ligand-induced splitting and complementary transmitted colour for full marks in explanation questions.

## Catalytic Activity of Transition Metals

Transition metals and their compounds are extremely common industrial and biological catalysts, for two key reasons:

- They can use their partially filled d orbitals to form temporary bonds with reactant molecules on the metal surface (heterogeneous catalysis)
- Their variable oxidation states allow them to participate in reaction steps that provide an alternative lower activation energy pathway (homogeneous catalysis)

- Haber process: iron catalyst
- Contact process: $V_2O_5$ catalyst
- Decomposition of $H_2O_2$: $MnO_2$ catalyst

**Check your understanding**

Test your understanding

1. Which of the following statements is correct?

   - All d-block elements are transition metals
   - Scandium is a transition metal because neutral Sc has a partially filled d orbital
   - Transition metals have variable oxidation states because 4s and 3d orbitals have similar energy
   - All transition metal ions are coloured

   *Answer:* Transition metals have variable oxidation states because 4s and 3d orbitals have similar energy

   *Why:* Correct! The similar energy of 4s and 3d orbitals allows different numbers of electrons to be lost, leading to variable oxidation states.

## Common pitfalls

- **Wrong:** Claiming scandium is a transition metal because neutral scandium has a partially filled d orbital
  - Why it fails: The definition of a transition metal is based on stable ion electron configuration, not neutral atom configuration
  - Correct: Scandium only forms $Sc^{3+}$, which has an empty 3d subshell, so it is not a transition metal
- **Wrong:** Stating that observed colour comes from light emitted by excited electrons
  - Why it fails: IB examiners expect you to attribute colour to transmitted complementary light after absorption, not emission
  - Correct: Specific wavelengths of visible light are absorbed for d-d transitions; the observed colour is the complementary transmitted light
- **Wrong:** Writing transition metal ion configurations by removing 3d electrons before 4s electrons
  - Why it fails: Once 3d orbitals start filling, 4s becomes higher in energy, so electrons are lost from 4s first
  - Correct: Always remove 4s electrons first when writing transition metal ion electron configurations
- **Wrong:** Claiming all d-block elements can form coloured ions
  - Why it fails: Only transition metals (with partially filled d ions) can have d-d transitions that absorb visible light
  - Correct: Non-transition d-block elements like scandium and zinc form only colourless ions

## Cheatsheet

| Category | d-block Element | Transition Metal |
| --- | --- | --- |
| Definition | Highest energy e- in d orbital; groups 3-12 | d-block with ≥1 stable ion with partially filled d subshell |
| Period 4 count | 10 total (Sc to Zn) | 8 out of 10 |
| Exceptions | N/A | Sc (only $Sc^{3+}: 3d^0$), Zn (only $Zn^{2+}: 3d^{10}$) |
| Variable oxidation states | N/A (only transition metals) | Yes, due to similar 4s/3d energy |
| Coloured ions | Only transition metals | Yes (for ions with partially filled d subshell) |
| Catalytic activity | N/A | Yes, from variable oxidation states and d electrons |

## What's next

Understanding d-block elements and transition metals is a foundational inorganic chemistry topic that underpins all more advanced transition metal content in IB HL Chemistry. This topic directly connects to coordination chemistry, where you will explore ligand bonding, crystal field splitting, and complex ion reactions, a major HL-only topic frequently assessed in Paper 2 and 3. It also links closely to redox chemistry, as variable oxidation states make transition metals central to redox titrations and electrochemical cell questions, which are common in both papers 1 and 2. Mastering the core definitions here will ensure you earn full marks on the many definition-based short answer questions that appear on IB exams.

- [Acid-base classification: Bronsted-Lowry and Lewis](https://www.owlsprep.com/study/ib-chemistry-hl-u3-acid-base-classification-bronsted-lowry/)
- [Salts and buffer solutions](https://www.owlsprep.com/study/ib-chemistry-hl-u3-salts-and-buffer-solutions/)
- [AHL: Extended periodic trends](https://www.owlsprep.com/study/ib-chemistry-hl-u3-ahl-extended-periodic-trends/)

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