Why Chromium and Copper Have Anomalous Electronic Configurations
Chemistry · D And F Block Elements · NEET
Chromium is 3d5 4s1 (not 3d4 4s2) and copper is 3d10 4s1 (not 3d9 4s2). One 4s electron shifts into 3d so the d subshell becomes exactly half-filled (d5) for Cr or completely filled (d10) for Cu. Half-filled and fully-filled d subshells have extra stability because the 3d and 4s orbitals are very close in energy. Memory hook: "Cr and Cu steal one 4s electron to make a happy d5 or d10."
One 4s electron shifts into 3d so Cr becomes 3d5 4s1 (half-filled) and Cu becomes 3d10 4s1 (fully filled) - both extra-stable because the 3d and 4s orbitals are close in energy.
Your doubts, answered
Why is chromium 3d5 4s1 and not 3d4 4s2?
Expected filling gives 3d4 4s2, but the real ground state is 3d5 4s1. One electron moves from 4s to 3d so the 3d subshell becomes exactly half-filled (d5). NCERT says the 3d and 4s orbitals are so close in energy that the small energy needed to move the electron is repaid by the extra stability of a half-filled d5 subshell. So Cr = [Ar] 3d5 4s1.
Why is copper 3d10 4s1 and not 3d9 4s2?
For copper, moving one 4s electron into 3d makes the d subshell completely filled (d10), which is very stable. So the ground state is 3d10 4s1, not 3d9 4s2. Same reason as chromium: the 3d and 4s energy gap is tiny, and a full d10 subshell is more stable. Cu = [Ar] 3d10 4s1.
Why are half-filled and fully-filled subshells extra stable?
Two reasons NEET wants: (1) Symmetry - a half-filled (d5) or full (d10) subshell has electrons spread evenly, giving a symmetric, low-energy arrangement. (2) Exchange energy - electrons with the same spin can swap places; more parallel-spin electrons means more exchange pairs and more exchange energy, which lowers the energy. d5 (five parallel spins) and d10 both maximise this stability.
What is exchange energy in simple words?
When two electrons in the same subshell have the same spin, they can exchange (swap) their positions. Each swap releases a small amount of energy called exchange energy. The more electrons with parallel spin, the more swaps possible, so the more stable the atom. A half-filled d5 has 5 parallel-spin electrons, giving the maximum number of exchanges for the 3d subshell - this is why d5 is specially stable.
Do only chromium and copper show this in the 3d series?
Yes. In the first (3d) transition series, only Cr (3d5 4s1) and Cu (3d10 4s1) break the normal Aufbau order. All other 3d elements follow the expected 4s2 filling. NEET often asks you to spot Cr and Cu as the two exceptions.
Does the same idea explain Gd, Eu, and Ce in the f-block?
Yes, the same stability rule appears with f orbitals. Eu is 4f7 6s2 and Gd is 4f7 5d1 6s2 - both keep the stable half-filled 4f7. Ce3+ (4f1) easily loses one more electron to become Ce4+ (empty 4f0), and Gd3+ reaches the stable 4f7. NEET reuses the 'half-filled / empty is stable' idea across d and f blocks.
⚠️ The NEET trap ✗ Writing chromium as 3d4 4s2 and copper as 3d9 4s2 because that is what plain Aufbau filling predicts. ✓ Chromium is [Ar] 3d5 4s1 and copper is [Ar] 3d10 4s1. One 4s electron shifts to 3d to give the extra-stable half-filled (d5) or fully-filled (d10) subshell. 🧠 If you see '4s2' for Cr or Cu, it is a trap - both keep only ONE 4s electron.
Real NEET questions
NEET 2024
The E° value for the Mn3+/Mn2+ couple is more positive than that of Cr3+/Cr2+ or Fe3+/Fe2+ due to the change of
A · d5 to d2 configuration
B · d4 to d5 configuration ✓
C · d3 to d5 configuration
D · d5 to d4 configuration
Solution: Mn3+ + e- -> Mn2+ changes Mn3+ (3d4) into Mn2+ (3d5). The product Mn2+ has the exceptionally stable half-filled 3d5 subshell, so this reduction is strongly favoured and E° is highly positive. This is the same half-filled stability rule that makes Cr's ground state 3d5 4s1. Correct option: (b) d4 to d5.
NEET 2022
Gadolinium has a low value of third ionisation enthalpy because of
A · small size
B · high exchange enthalpy ✓
C · high electronegativity
D · high basic character
Solution: Gadolinium is [Xe] 4f7 5d1 6s2. Removing the third electron gives Gd3+, which reaches the extra-stable half-filled 4f7 arrangement. A half-filled subshell has large exchange energy (high exchange enthalpy), so forming Gd3+ is easy and the third ionisation enthalpy is unusually low. Exchange energy is the same factor behind Cr's 3d5 4s1 configuration. Correct option: (b).
NEET 2016 Phase 1
The electronic configurations of Eu (Z=63), Gd (Z=64) and Tb (Z=65) are:
A · [Xe] 4f7 6s2, [Xe] 4f8 6s2 and [Xe] 4f8 5d1 6s2
B · [Xe] 4f6 5d1 6s2, [Xe] 4f7 5d1 6s2 and [Xe] 4f9 6s2
C · [Xe] 4f6 5d1 6s2, [Xe] 4f7 5d1 6s2 and [Xe] 4f8 5d1 6s2
D · [Xe] 4f7 6s2, [Xe] 4f7 5d1 6s2 and [Xe] 4f9 6s2 ✓
Solution: Eu keeps the stable half-filled 4f7: [Xe] 4f7 6s2. Gd cannot break the half-filled 4f7, so the extra electron goes to 5d: [Xe] 4f7 5d1 6s2 (an anomaly just like Cr/Cu keeping a stable subshell). Tb is [Xe] 4f9 6s2. Correct option: (d). This shows the same half-filled stability seen in chromium.
Solved D And F Block Elements NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the correct electronic configuration of chromium?
Chromium (Z=24) is [Ar] 3d5 4s1, not [Ar] 3d4 4s2. One 4s electron moves to 3d to give a stable half-filled d5 subshell.
What is the correct electronic configuration of copper?
Copper (Z=29) is [Ar] 3d10 4s1, not [Ar] 3d9 4s2. One 4s electron moves to 3d to give a stable completely-filled d10 subshell.
Which two 3d elements have anomalous configurations?
Only chromium (3d5 4s1) and copper (3d10 4s1) are exceptions in the 3d series. All other first-row transition metals follow normal 4s2 filling.
Why do half-filled and full d subshells give extra stability?
Because of symmetric electron distribution and maximum exchange energy. More parallel-spin electrons means more exchanges, which lowers energy - d5 and d10 are especially stable.
Why does this matter for NEET?
NEET regularly tests these two exceptions and the same half-filled/fully-filled stability idea in ionisation enthalpy, oxidation state (Mn2+ d5), and lanthanoid (Gd, Eu, Ce) questions. Knowing it lets you answer several question types with one rule.
