d-block elements and transition metals

IB Chemistry HL· Topic 3.3· 15 min read

1. Definitions: d-block vs Transition Metals★★☆☆☆⏱ 4 min

📘 Definition

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

📘 Definition

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 $S c^{3 +}$ (empty d), Zn only forms $Z n^{2 +}$ (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.

2. Characteristic Properties: Variable Oxidation States★★☆☆☆⏱ 5 min

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.

📘 Definition

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 $[F e (C N)_{6}]^{3 -}$ and Mn in $K M n O_{4}$ .

1
For $[F e (C N)_{6}]^{3 -}$ : each $C N^{-}$ ligand has a charge of -1. Let x = oxidation state of Fe:
2
$x + (6 \times -1) = -3 \implies x = +3$
3
For $K M n O_{4}$ : 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

3. Origin of Colour in Transition Metal Ions★★★☆☆⏱ 5 min

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.

📐 Worked Example

Explain why aqueous $C u S O_{4}$ is blue, while aqueous $Z n S O_{4}$ 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 $C u^{2 +}$ . $C u^{2 +}$ 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
$Z n^{2 +}$ 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.

4. Catalytic Activity of Transition Metals★★★☆☆⏱ 4 min

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: $V_{2} O_{5}$ catalyst
Decomposition of $H_{2} O_{2}$ : $M n O_{2}$ catalyst

✓ Quick check

Test your understanding

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
Reveal answer
2 —
Correct! The similar energy of 4s and 3d orbitals allows different numbers of electrons to be lost, leading to variable oxidation states.

5. Common Pitfalls

Wrong move:

Claiming scandium is a transition metal because neutral scandium has a partially filled d orbital

Why:

The definition of a transition metal is based on stable ion electron configuration, not neutral atom configuration

Correct move:

Scandium only forms $S c^{3 +}$ , which has an empty 3d subshell, so it is not a transition metal

Wrong move:

Stating that observed colour comes from light emitted by excited electrons

Why:

IB examiners expect you to attribute colour to transmitted complementary light after absorption, not emission

Correct move:

Specific wavelengths of visible light are absorbed for d-d transitions; the observed colour is the complementary transmitted light

Wrong move:

Writing transition metal ion configurations by removing 3d electrons before 4s electrons

Why:

Once 3d orbitals start filling, 4s becomes higher in energy, so electrons are lost from 4s first

Correct move:

Always remove 4s electrons first when writing transition metal ion electron configurations

Wrong move:

Claiming all d-block elements can form coloured ions

Why:

Only transition metals (with partially filled d ions) can have d-d transitions that absorb visible light

Correct move:

Non-transition d-block elements like scandium and zinc form only colourless ions

6. Quick Reference 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 $S c^{3 +} : 3 d^{0}$ ), Zn (only $Z n^{2 +} : 3 d^{10}$ )
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

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2025 · P2
Distinguish d-block vs transition metals
2024 · P1
Identify transition metal examples
2023 · P2
Explain coloured ion formation

Going deeper

syllabusIB Chemistry 2025 Official Topic 3Official syllabus overview

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.