Transition Elements: Definition and Electron Configurations

Detailed Explanations

A. Why 4s is removed before 3d (what to write)

When electrons are removed, the 4s electrons are lost first because after the 3d subshell begins to fill, the 4s electrons are higher in energy (and more easily removed) in the ions; therefore transition-metal cations are written without 4s electrons.

B. First-row configurations (core patterns)

General pattern:

• [Ar] 3dx 4s² for many elements

Common exceptions:

• chromium: Cr is [Ar] 3d⁵ 4s¹
• copper: Cu is [Ar] 3d¹⁰ 4s¹

C. Workflow: writing electron configurations for ions (exam method)

1. Write the electron configuration for the neutral atom (use the Cr/Cu exceptions).
2. Remove electrons from 4s first, then from 3d.
3. Rewrite the ion configuration (typically with no 4s electrons for first-row transition-metal cations).

Mini example: Start with Fe: [Ar] 3d⁶ 4s². For Fe³⁺, remove 2 electrons from 4s and 1 from 3d → [Ar] 3d⁵.

D. Forming ions (worked patterns)

Example: iron

• Fe is [Ar] 3d⁶ 4s²
• Fe²⁺ is [Ar] 3d⁶
• Fe³⁺ is [Ar] 3d⁵

Example: copper

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

E. Why radii and first ionisation energies vary only slightly

Across the first transition series, proton number increases, but each additional electron enters the inner 3d subshell and increases shielding. The effective attraction felt by the outer electrons therefore changes less than a simple increase in nuclear charge suggests. State relatively invariant: small irregular changes remain because 3d electron pairing and subshell stability also vary.

F. Melting point and density compared with calcium

Most first-row transition elements have higher melting points than calcium because both 4s and available 3d electrons can contribute to stronger metallic bonding. They also have higher densities: their atoms have greater mass while their metallic radii do not increase correspondingly. This is a qualitative comparison; do not claim every value is identical across the series.