Chapter 4 · Chemistry
d-Block Configurations: Why Chromium and Copper Break the Rule
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The d-block's general configuration is (n−1)d¹⁻¹⁰ ns¹⁻², with 4s filling before 3d. Chromium (3d⁵4s¹) and copper (3d¹⁰4s¹) move one 4s electron into 3d, because 3d and 4s are very close in energy and half-filled and full d sets are extra stable.
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Why Transition Metals Have High Melting PointsIn transition metals the (n−1)d electrons join the ns electrons in metallic bonding, so the bonds are strong and the metals are hard with high melting points. Across the 3d series melting points peak near the middle (Mn is an exception) and fall to zinc (d¹⁰).Chemistry
Key points
- General outer configuration: (n−1)d¹⁻¹⁰ ns¹⁻²; 4s fills before 3d.
- Cr: 3d⁵ 4s¹ (not 3d⁴ 4s²): a half-filled d set.
- Cu: 3d¹⁰ 4s¹ (not 3d⁹ 4s²): a completely filled d set.
- Why: 3d and 4s are very close in energy, and d⁵ and d¹⁰ are extra stable (exchange energy, symmetry).
The 3d series
| Element | Configuration | Element | Configuration |
|---|---|---|---|
| Sc | [Ar] 3d¹ 4s² | Fe | [Ar] 3d⁶ 4s² |
| Ti | [Ar] 3d² 4s² | Co | [Ar] 3d⁷ 4s² |
| V | [Ar] 3d³ 4s² | Ni | [Ar] 3d⁸ 4s² |
| Cr | [Ar] 3d⁵ 4s¹ | Cu | [Ar] 3d¹⁰ 4s¹ |
| Mn | [Ar] 3d⁵ 4s² | Zn | [Ar] 3d¹⁰ 4s² |
Why chromium and copper are different
- The 3d and 4s orbitals are very close in energy, so a small extra stability decides the arrangement.
- Half-filled (d⁵) and completely filled (d¹⁰) sets are extra stable: more exchange energy and a symmetrical distribution.
- So one 4s electron moves into 3d: Cr 3d⁵4s¹, Cu 3d¹⁰4s¹.
In the 4d series palladium goes further: 4d¹⁰ 5s⁰.
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More from The d- and f-Block Elements
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