# Abnormal electronic configuration of chromium and copper

> Why Cr and Cu break the pattern


## Objectives

- Write the configurations of Cr (Z = 24) and Cu (Z = 29) as the book writes them.
- Give the book's reason: the atom has lower energy (extra stability) when d is half filled ($d^5$) or filled ($d^{10}$).
- Recognize that in the first transition series only Cr and Cu are anomalous.

## 1. Electronic configurations and oxidation states




## 2. One electron more than vanadium

Predict: how does the book write chromium?

- $[\text{Ar}]\ 4s^1\ 3d^5$ ✓
- $[\text{Ar}]\ 4s^2\ 3d^4$
- $[\text{Ar}]\ 4s^2\ 3d^3\ 4p^1$

By the general pattern Cr would be $[\text{Ar}]\ 4s^2\ 3d^4$, but the book writes $[\text{Ar}]\ 4s^1\ 3d^5$: one electron sits in 3d instead of 4s, so 3d is half filled.

[ar-EG] على القاعدة العامة الـ Cr كان هيبقى $[\text{Ar}]\ 4s^2\ 3d^4$، لكن الكتاب بيكتبه $[\text{Ar}]\ 4s^1\ 3d^5$: electron واحد بيبقى في الـ 3d بدل الـ 4s، فالـ 3d بيبقى half filled.
## 3. Chromium: expected vs real

Expected pattern $4s^2\ 3d^4$: one 3d orbital stays empty Real Cr (the book) $\ce{_{24}Cr}$: $[\text{Ar}]\ 4s^1\ 3d^5$ 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 $3d^5$ or $4s^1$. In Cr, both 4s ($4s^1$) and 3d ($3d^5$) are half filled.


## 4. Move one electron in copper

Copper (Z = 29) has 11 electrons after $[\text{Ar}]$. Put 2 or 1 of them in 4s, and the rest in 3d. electrons in 4s $4s^2\ 3d^9$ (the expected pattern): one 3d orbital has an unpaired electron $4s^1\ 3d^{10}$: every 3d orbital holds a pair This is the real copper atom $\ce{_{29}Cu}$: $[\text{Ar}]\ 4s^1\ 3d^{10}$. Here 4s is half filled, but 3d is filled ($d^{10}$). Filled sublevel A sublevel in which every orbital holds two electrons, like $3d^{10}$.


## 5. The book's reason

Why do Cr and Cu do this? The book gives one reason, in three links: Book p.9: why Cr and Cu are anomalous Extra stability The lower energy an atom has when its d sublevel is half filled ($d^5$) or filled ($d^{10}$). The book adds This is not the only main factor that causes the stability of the atom.


## 6. Which are written correctly?

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


## 7. Silver, and Cr in full

Silver ($\ce{_{47}Ag}$) is below copper in group IB. Its outer sublevels are $5s^1\ 4d^{10}$. Which reason fits?

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

Book p.10 lists Cu ($4s^1\ 3d^{10}$), Ag ($5s^1\ 4d^{10}$) and Au ($6s^1\ 5d^{10}$): each has a filled d sublevel.

[ar-EG] الكتاب في صفحة 10 بيذكر الـ Cu ($4s^1\ 3d^{10}$) والـ Ag ($5s^1\ 4d^{10}$) والـ Au ($6s^1\ 5d^{10}$): كل واحد فيهم عنده d sublevel مليان (filled).
## 8. Summary

Chromium: 3d half filled ($d^5$) $\ce{_{24}Cr}$: $[\text{Ar}]\ 4s^1\ 3d^5$ Copper: 3d filled ($d^{10}$) $\ce{_{29}Cu}$: $[\text{Ar}]\ 4s^1\ 3d^{10}$ $\ce{_{24}Cr}$: $[\text{Ar}]\ 4s^1\ 3d^5$: 4s and 3d are both half filled. $\ce{_{29}Cu}$: $[\text{Ar}]\ 4s^1\ 3d^{10}$: 4s is half filled, 3d is filled. In the first series only Cr and Cu are anomalous. Give reasons: the abnormal configuration of Cr and Cu Because the atom has **lower energy** (**extra stability**) when d is half filled ($d^5$), as in Cr, or filled ($d^{10}$), as in Cu. Back to the hook: Cr is $4s^1\ 3d^5$ because a half-filled 3d lowers the atom's energy. Can you explain why copper ends with $4s^1\ 3d^{10}$ and not $4s^2\ 3d^9$?



Chapter: Transition Elements (https://yaani.ai/ar/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/ar/lesson/chem-s3-en.lesson-te.cr-cu-anomaly