Transition Elements: Definition and Properties

Learning Objectives (OCR Spec 5.3.1)

By the end of this lesson you should be able to:

• State the definition of a transition element in terms of d-electron configuration
• Explain why scandium and zinc are d-block but not transition elements
• List the four characteristic properties of transition elements
• Explain qualitatively why transition elements show these properties
• Distinguish between a d-block element and a transition element

The OCR Definition

A transition element is a d-block element that forms at least one stable ion with a partially filled d sub-shell.

This is a precise definition and you must learn it word-for-word. Three parts matter:

1. The element sits in the d-block of the Periodic Table (3d, 4d or 5d series).
2. It must form at least one ion (not necessarily the neutral atom) with d-electrons.
3. That ion must have a partially filled d sub-shell - meaning between 1 and 9 d-electrons inclusive.

The First Row (3d Series): Ti to Cu

At A-Level OCR focuses almost exclusively on the first row of the d-block (Period 4, atomic numbers 21-30). The ten elements are:

Z	Symbol	Element	Transition element?
21	Sc	Scandium	No (only Sc3+ which is d0)
22	Ti	Titanium	Yes
23	V	Vanadium	Yes
24	Cr	Chromium	Yes
25	Mn	Manganese	Yes
26	Fe	Iron	Yes
27	Co	Cobalt	Yes
28	Ni	Nickel	Yes
29	Cu	Copper	Yes
30	Zn	Zinc	No (only Zn2+ which is d10)

Scandium and zinc are excluded because they do not form any ion with a partially filled d-subshell. Scandium only forms Sc3+ (configuration [Ar], no d-electrons, d0). Zinc only forms Zn2+ (configuration [Ar] 3d10, completely full d-subshell). Both are d-block elements (their highest-energy electron goes into the 3d) but neither is a transition element by the OCR definition.

graph TD
  A[d-block element] --> B{Forms at least one ion<br/>with partially filled d?}
  B -->|Yes| C[Transition element<br/>Ti, V, Cr, Mn, Fe, Co, Ni, Cu]
  B -->|No| D[d-block but NOT transition<br/>Sc d0, Zn d10]

The Four Characteristic Properties

Transition elements show four key properties that make them different from s-block and p-block metals:

1. Variable oxidation states (they form ions in more than one oxidation state)
2. Coloured compounds and ions in solution
3. Catalytic activity (as the element, its oxide, or a salt)
4. Form complex ions with ligands

These properties all arise from the same underlying electronic feature: the energy of the 3d sub-shell is very close to that of the 4s sub-shell, so electrons can be added and removed from 3d without large energy changes, and the incomplete 3d sub-shell allows absorption of visible light.

graph TD
  A[Transition metal<br/>partially filled 3d] --> B[Variable oxidation states]
  A --> C[Coloured ions]
  A --> D[Catalysis]
  A --> E[Complex ion formation]
  B --> F[3d and 4s close in energy]
  C --> G[d-d electron transitions<br/>absorb visible light]
  D --> H[Provide alternative pathway<br/>variable ox states allow<br/>electron transfer]
  E --> I[Small highly charged cations<br/>with empty 3d/4s/4p orbitals<br/>accept lone pairs]

Property 1: Variable Oxidation States

Because the 3d and 4s orbitals are very close in energy, the electrons in both can be removed in steps. This gives rise to multiple oxidation states.

Element	Common oxidation states
Ti	+2, +3, +4
V	+2, +3, +4, +5
Cr	+2, +3, +6
Mn	+2, +3, +4, +6, +7
Fe	+2, +3
Co	+2, +3
Ni	+2
Cu	+1, +2

The most common (or most stable) states are highlighted in bold for reference. Notice the pattern: oxidation states generally rise to a maximum in the middle of the row (Mn +7, matching the number of 3d + 4s electrons) and then fall back because at higher Z the remaining 3d electrons are pulled too tightly to the nucleus to ionise.

Property 2: Coloured Ions

Most transition metal ions are coloured in solution or in solid compounds. A non-exhaustive list:

Ion	Formula (aqueous)	Colour
Titanium(III)	[Ti(H2O)6]3+	Purple
Vanadium(II)	[V(H2O)6]2+	Violet
Vanadium(III)	[V(H2O)6]3+	Green
Vanadium(IV)	VO2+	Blue
Vanadium(V)	VO2+ (dioxovanadium)	Yellow
Chromium(III)	[Cr(H2O)6]3+	Green / violet
Chromium(VI)	CrO4^2- / Cr2O7^2-	Yellow / orange
Manganese(II)	[Mn(H2O)6]2+	Very pale pink
Manganese(VII)	MnO4-	Deep purple
Iron(II)	[Fe(H2O)6]2+	Pale green
Iron(III)	[Fe(H2O)6]3+	Pale yellow / brown
Cobalt(II)	[Co(H2O)6]2+	Pink
Nickel(II)	[Ni(H2O)6]2+	Green
Copper(II)	[Cu(H2O)6]2+	Pale blue

The colours arise from d-d transitions - when the complex absorbs a photon in the visible region, an electron is promoted from a lower-energy d-orbital to a higher-energy d-orbital. The colour we see is the complementary colour of the light absorbed. Sc3+ (d0) and Zn2+ (d10) have no d-d transitions available (no empty or no filled orbitals respectively) and so their compounds are colourless.

Property 3: Catalytic Activity

Transition elements and their compounds are excellent catalysts because they can easily change oxidation state and bind substrate molecules on empty d-orbital sites. Key examples you must know:

Catalyst	Reaction	Type
Fe(s)	Haber process: N2 + 3H2 -> 2NH3	Heterogeneous
V2O5	Contact process: 2SO2 + O2 -> 2SO3	Heterogeneous
Ni(s)	Hydrogenation of alkenes	Heterogeneous
Pt / Rh / Pd	Catalytic converters in car exhausts	Heterogeneous
MnO2	2H2O2 -> 2H2O + O2	Heterogeneous
Fe2+ / Fe3+	S2O8^2- + 2I- -> 2SO4^2- + I2	Homogeneous
Co2+	Oxidation of tartrate by H2O2 (colour change indicator)	Homogeneous (autocatalysis)

Property 4: Complex Ion Formation

A small, highly charged transition metal cation with empty d, s and p orbitals can accept lone pairs from surrounding molecules or ions called ligands. The resulting structure is a complex ion. Formation of complex ions will be covered in detail in lessons 3-7, but the quick example is:

• [Cu(H2O)6]2+ - hexaaquacopper(II), pale blue, octahedral
• [CuCl4]2- - tetrachlorocuprate(II), yellow, tetrahedral
• [Cu(NH3)4(H2O)2]2+ - tetraamminediaquacopper(II), deep royal blue, octahedral

Why Are 3d and 4s Close in Energy?

The 4s orbital penetrates the 1s, 2s, 2p, 3s, 3p core and reaches the nucleus - it is slightly more stable than the 3d orbital, which does not penetrate as effectively. However, the difference is small, and as electrons enter the 3d the nuclear attraction becomes stronger; by the time we reach Cu and Zn, the 3d orbitals are actually lower in energy than the 4s. This is why when we form ions, we always remove the 4s electrons first, even though the Aufbau order filled 4s before 3d.

For example: Fe [Ar] 3d6 4s2 -> Fe2+ [Ar] 3d6 (NOT [Ar] 3d4 4s2).

This is the single most important rule for writing d-block electron configurations, and the focus of lesson 2.

Exam Tips

• Learn the OCR definition word-perfect: "A transition element is a d-block element that forms at least one stable ion with a partially filled d sub-shell."
• Always state both reasons Sc and Zn are not transition metals: Sc3+ is d0, Zn2+ is d10.
• When listing characteristic properties, give a reason alongside each (3d energy, d-d transitions, variable state, empty d orbitals).
• Practice writing equations for the catalyst reactions above. The Contact process and Haber process come up regularly.
• Match colours to the exact formula - "Cu2+ is blue" is vague; [Cu(H2O)6]2+ is pale blue but [Cu(NH3)4(H2O)2]2+ is deep royal blue.

Common Exam Mistakes

• Saying Sc or Zn are "not in the d-block" - they are; they simply are not transition elements.
• Forgetting the word stable in the definition (some candidates write "forms at least one ion with d-electrons" which is loose).
• Confusing "coloured compound" with "paramagnetic" - colour comes from d-d transitions specifically.
• Quoting "because they are metals" as the reason for catalytic activity - the correct reason is variable oxidation states and available d-orbitals.
• Writing "Cu2+ has d9, so it is coloured because d-d transitions happen" but giving no mention of ligand field splitting.

In lesson 2 we will look in detail at the electron configurations of the first-row transition metals, including the famous anomalies of Cr and Cu.