Abnormal electronic configuration of chromium and copper

Why Cr and Cu break the pattern

1. Electronic configurations and oxidation states

Abnormal electronic configuration of chromium and copper

2. One electron more than vanadium

Vanadium (Z = 23) is

[Ar]\ 4s²\ 3d³

. Chromium (Z = 24) has one electron more.

Pick the right answer

Predict: how does the book write chromium?

A. $[Ar]\ 4s²\ 3d⁴$
B. $[Ar]\ 4s¹\ 3d⁵$
C. $[Ar]\ 4s²\ 3d³\ 4p¹$

Explanation

By the general pattern Cr would be

[Ar]\ 4s²\ 3d⁴

, but the book writes

[Ar]\ 4s¹\ 3d⁵

: one electron sits in 3d instead of 4s, so 3d is half filled.

3. Chromium: expected vs real

Expected pattern

↑↓

↑↑↑↑

4s²\ 3d⁴

: one 3d orbital stays empty

Real Cr (the book)

↑

↑↑↑↑↑

₂₄Cr X_{24} X_{224}^{2} Cr

[Ar]\ 4s¹\ 3d⁵

Compared with the pattern, 4s has one electron less and 3d one more. Now each of the five 3d orbitals holds one electron.

Half-filled sublevel

A sublevel in which every orbital holds one electron, like

3d⁵ 3 d^{5}

4s¹ 4 s^{1}

In Cr, both 4s (

4s¹ 4 s^{1}

) and 3d (

3d⁵ 3 d^{5}

) are half filled.

4. Move one electron in copper

Copper (Z = 29) has 11 electrons after

[Ar] [Ar]

. Put 2 or 1 of them in 4s, and the rest in 3d.

electrons in 4s 2

↑↓

↑↓↑↓↑↓↑↓↑

4s²\ 3d⁹

(the expected pattern): one 3d orbital has an unpaired electron

5. The book's reason

Why do Cr and Cu do this? The book gives one reason, in three links:

3d half filled (d⁵) in Cr, or filled (d¹⁰) in Cu → The atom has lower energy
The atom has lower energy → Extra stability

Book p.9: why Cr and Cu are anomalous

Extra stability

The lower energy an atom has when its d sublevel is half filled (

d⁵ d^{5}

) or filled (

d¹⁰ d^{10}

6. Which are written correctly?

Sort into groups

Sort each atom: is it written the way the book writes it?

$₂₅Mn X_{25} X_{225}^{2} Mn$ : $[Ar]\ 4s¹\ 3d⁶$ $₂₇Co X_{27} X_{227}^{2} Co$ : $[Ar]\ 4s²\ 3d⁷$ $₂₆Fe X_{26} X_{226}^{2} Fe$ : $[Ar]\ 4s¹\ 3d⁷$ $₂₄Cr X_{24} X_{224}^{2} Cr$ : $[Ar]\ 4s²\ 3d⁴$ $₂₈Ni X_{28} X_{228}^{2} Ni$ : $[Ar]\ 3d¹⁰$ $₂₉Cu X_{29} X_{229}^{2} Cu$ : $[Ar]\ 4s¹\ 3d¹⁰$

Need a hint?

Only two atoms of this series are anomalous.
An atom moves to $4s¹ 4 s^{1}$ only if 3d then becomes half filled or filled.
Check each 3d count: is it 5, 10, or neither?

7. Silver, and Cr in full

Pick the right answer

Silver (

₄₇Ag X_{47} X_{247}^{2} Ag

) is below copper in group IB. Its outer sublevels are

5s¹\ 4d¹⁰

. Which reason fits?

A. A filled d sublevel ( $d¹⁰ d^{10}$ ) gives the atom lower energy, so extra stability
B. An s orbital can hold only one electron
C. A half-filled d sublevel ( $d⁵ d^{5}$ ) gives extra stability
D. Bigger atoms put more electrons in s

Explanation

Book p.10 lists Cu (

4s¹\ 3d¹⁰

), Ag (

5s¹\ 4d¹⁰

) and Au (

6s¹\ 5d¹⁰

): each has a filled d sublevel.

Fill in the blanks

Full configuration:

₂₄Cr X_{24} X_{224}^{2} Cr

1s²\ 2s²\ 2p⁶\ 3s²\ 3p⁶

4s_____ 3d_____ (type the exponents)

Explanation

₂₄Cr X_{24} X_{224}^{2} Cr

1s²\ 2s²\ 2p⁶\ 3s²\ 3p⁶\ 4s¹\ 3d⁵

: both 4s and 3d are half filled.

8. Summary

Chromium: 3d half filled (

d⁵ d^{5}

)

↑

↑↑↑↑↑

₂₄Cr X_{24} X_{224}^{2} Cr

[Ar]\ 4s¹\ 3d⁵

Copper: 3d filled (

d¹⁰ d^{10}

)

↑

↑↓↑↓↑↓↑↓↑↓

₂₉Cu X_{29} X_{229}^{2} Cu

[Ar]\ 4s¹\ 3d¹⁰

$₂₄Cr X_{24} X_{224}^{2} Cr$ : $[Ar]\ 4s¹\ 3d⁵$ : 4s and 3d are both half filled.
$₂₉Cu X_{29} X_{229}^{2} Cu$ : $[Ar]\ 4s¹\ 3d¹⁰$ : 4s is half filled, 3d is filled.
In the first series only Cr and Cu are anomalous.

Back to the hook: Cr is

4s¹\ 3d⁵

because a half-filled 3d lowers the atom's energy.

Can you explain why copper ends with

4s¹\ 3d¹⁰

and not

4s²\ 3d⁹

Concepts in this lesson