Period 4 d-block elements display characteristic electron configurations that underpin their chemistry. Understanding sub-shell notation allows prediction of atomic and ionic structures essential for later transition-metal topics.

Electron Configurations of Period 4 d-Block Elements

Period 4 contains the ten d-block elements from scandium (Sc) to zinc (Zn). In the OCR A-Level course, students must be able to write their electron configurations accurately in sub-shell notation, including the configurations of their common ions. These configurations provide the foundation for explaining their variable oxidation states, coloured ions, and complex formation encountered later in the transition-metal topic.

Before examining individual atoms and ions, it is essential to recognise how electrons fill the 4s and 3d sub-shells.

Sub-Shell Filling Order

Electrons occupy orbitals in order of increasing energy. Period 4 d-block elements follow a characteristic filling pattern: the 4s orbital is filled before the 3d orbitals when writing configurations for neutral atoms. This ordering is crucial for correct notation.

This diagram shows the orbital-filling order used to write electron configurations. Following the diagonal arrows gives the standard sequence, including 4s filling before 3d in Period 4. Source

The filling pattern continues through the series as electrons populate the five 3d orbitals. Each 3d orbital can hold a pair of electrons, giving a total of ten electrons in the 3d sub-shell.

Key features of filling behaviour

• The 4s sub-shell is always written before 3d in configuration notation for neutral atoms because it is filled first.

• The 3d sub-shell fills progressively from Sc (3d¹) to Zn (3d¹⁰).

• Electrons obey the Pauli Exclusion Principle and Hund’s Rule, ensuring correct arrangement within orbitals, though detailed orbital diagrams are not required by the specification.

A further important point emerges when writing configurations for ions, especially those of transition metals.

Electron Removal in Ion Formation

When Period 4 d-block elements form ions, the energy ordering shifts so that electrons are removed from the 4s sub-shell before the 3d sub-shell. This point is frequently tested and must be emphasised.

Even though the 4s fills first, it is always the first to lose electrons during ionisation. This reversal highlights that sub-shell energy ordering differs between isolated atoms and their ions.

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Rules for writing ion configurations

• Write the neutral atom configuration first.

• Remove electrons from 4s before 3d.

• Ensure sub-shell notation remains in the order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p (even though 4s electrons are lost first).

This distinction becomes especially significant for elements such as titanium, vanadium, chromium, manganese, and copper, which form multiple oxidation states.

Period 4 d-Block Elements: Atomic Electron Configurations

The neutral atoms follow a broadly predictable pattern. Students must know how to derive these using the Aufbau principle and apply sub-shell notation correctly.

General trends across the series

• Sc (Z=21): 4s² 3d¹

• Ti (Z=22): 4s² 3d²

• V (Z=23): 4s² 3d³

• Cr (Z=24): Exception – 4s¹ 3d⁵ (stabilised half-filled d sub-shell)

• Mn (Z=25): 4s² 3d⁵

• Fe (Z=26): 4s² 3d⁶

• Co (Z=27): 4s² 3d⁷

• Ni (Z=28): 4s² 3d⁸

• Cu (Z=29): Exception – 4s¹ 3d¹⁰ (stabilised full d sub-shell)

• Zn (Z=30): 4s² 3d¹⁰

This diagram summarises the valence electron configurations of the Period 4 transition metals using [Ar] 3dⁿ 4sᵐ notation. It highlights the chromium and copper exceptions where a 4s electron is promoted for added d-subshell stability. Source

Notes on the exceptions

• Chromium and copper depart from the expected 4s² 3d⁴ and 4s² 3d⁹ patterns.

• Half-filled (d⁵) and fully filled (d¹⁰) sub-shells offer added stability due to symmetrical electron distribution and reduced electron repulsion.

• These exceptions are required knowledge for accurate atomic and ionic configuration writing.

These orbital diagrams illustrate how electrons are distributed between 4s and 3d orbitals for early Period 4 transition metals. Arrow direction shows electron spin, which is additional detail beyond the syllabus minimum, while the written configurations emphasise the chromium 3d⁵ 4s¹ exception. Source

Period 4 d-Block Ions: Electron Configurations

Students must also write configurations for common ions such as Sc³⁺, Ti³⁺/Ti²⁺, V³⁺/V²⁺, Cr³⁺, Mn²⁺, Fe²⁺/Fe³⁺, Co²⁺, Ni²⁺, Cu⁺/Cu²⁺, and Zn²⁺.
Although the exact list is not specified, these ions commonly appear across OCR questions.

General principles

• Remove the 4s electrons first (up to two electrons).

• Then remove electrons from the 3d sub-shell as needed to match the ionic charge.

Examples of typical patterns (non-worked, structural only)

• Ti: Ti → [Ar] 4s² 3d² → Ti³⁺: [Ar] 3d¹

• Mn: Mn → [Ar] 4s² 3d⁵ → Mn²⁺: [Ar] 3d⁵

• Fe: Fe → [Ar] 4s² 3d⁶ → Fe³⁺: [Ar] 3d⁵

• Cu: Cu → [Ar] 4s¹ 3d¹⁰ → Cu²⁺: [Ar] 3d⁹

• Zn: Zn → [Ar] 4s² 3d¹⁰ → Zn²⁺: [Ar] 3d¹⁰

These structural patterns help students recognise which ions maintain partially filled d sub-shells and therefore meet the OCR definition of a transition element.

Why Electron Configurations Matter in Later Topics

Understanding electron configurations for Period 4 d-block elements is essential preparation for the remainder of Topic 11. Ion configurations underpin:

• Variable oxidation states (11.1.3)

• Coloured ions arising from d–d transitions (11.1.3)

• Formation of complex ions with ligands (11.1.4)

• Ligand substitution and stability in biological systems (11.1.6)

Although these ideas appear in later sections, they rely directly on the ability to write configurations in correct sub-shell notation as required in this subsubtopic.