Transition Metal Properties

Transition metals (AQA 3.2.5) are d-block elements that form at least one stable ion with a partially filled d sub-shell. This lesson covers their electron configurations, the definition of a transition metal, variable oxidation states, coloured ion formation, and catalytic activity.

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Definition

Key Definition: A transition metal is a d-block element that forms one or more stable ions with an incomplete d sub-shell.

This definition is important because it excludes scandium and zinc:

• Scandium (Sc): [Ar] 3d¹4s². Its only ion is Sc³⁺, which has the configuration [Ar] — an empty 3d sub-shell, not a partially filled one.
• Zinc (Zn): [Ar] 3d¹⁰ 4s². Its only ion is Zn²⁺, which has the configuration [Ar] 3d¹⁰ — a full 3d sub-shell, not a partially filled one.

Exam Tip: Always use the precise definition. "d-block element" and "transition metal" are NOT the same thing. Sc and Zn are d-block elements but not transition metals.

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Electron Configurations

Atoms

The 4s sub-shell fills before the 3d sub-shell in atoms (4s is lower in energy in neutral atoms):

Element	Symbol	Atomic number	Electron configuration
Scandium	Sc	21	[Ar] 3d¹4s²
Titanium	Ti	22	[Ar] 3d²4s²
Vanadium	V	23	[Ar] 3d³4s²
Chromium	Cr	24	[Ar] 3d⁵ 4s¹
Manganese	Mn	25	[Ar] 3d⁵ 4s²
Iron	Fe	26	[Ar] 3d⁶ 4s²
Cobalt	Co	27	[Ar] 3d⁷ 4s²
Nickel	Ni	28	[Ar] 3d⁸ 4s²
Copper	Cu	29	[Ar] 3d¹⁰ 4s¹
Zinc	Zn	30	[Ar] 3d¹⁰ 4s²

Anomalous Configurations

Chromium is [Ar] 3d⁵ 4s¹ (not 3d⁴ 4s²) because a half-filled 3d sub-shell (3d⁵) provides extra stability due to the symmetrical distribution of electrons (exchange energy).

Copper is [Ar] 3d¹⁰ 4s¹ (not 3d⁹ 4s²) because a fully filled 3d sub-shell (3d¹⁰) provides extra stability.

Ions

When transition metals form ions, the 4s electrons are lost first (4s is higher in energy in ions than 3d):

Ion	Configuration
Ti²⁺	[Ar] 3d²
V³⁺	[Ar] 3d²
Cr³⁺	[Ar] 3d³
Mn²⁺	[Ar] 3d⁵
Fe²⁺	[Ar] 3d⁶
Fe³⁺	[Ar] 3d⁵
Co²⁺	[Ar] 3d⁷
Ni²⁺	[Ar] 3d⁸
Cu⁺	[Ar] 3d¹⁰
Cu²⁺	[Ar] 3d⁹

Common Misconception: Students often write the electron configuration of Fe²⁺ as [Ar] 3d⁴ 4s² (removing from 3d first). This is wrong. Always remove 4s electrons first when forming ions.

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Variable Oxidation States

Transition metals exhibit variable oxidation states because the 3d and 4s energy levels are close together, so different numbers of electrons can be involved in bonding.

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

Colours of Common Ions in Aqueous Solution