# Electronic Configuration and Aufbau Principle

> Electronic configuration and Aufbau principle for NEET Chemistry: n plus l rule, Hund's rule, Cr and Cu exceptions, ion configurations.

- Canonical URL: https://prepelephant.com/topics/neet-ug/chemistry/electronic-configuration-and-aufbau
- Exam / course: NEET-UG · Subject: Chemistry
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Electronic Configuration and Aufbau Principle", PrepElephant, https://prepelephant.com/topics/neet-ug/chemistry/electronic-configuration-and-aufbau

## Direct answer

Electrons fill orbitals in order of rising energy — 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p — the Aufbau principle, decided by the lower (n + l) rule with lower n winning any tie. Within a subshell, Hund's rule of maximum multiplicity places electrons singly with parallel spins before any pairing, and Pauli's exclusion caps every orbital at two electrons of opposite spin. Two ground-state exceptions must be memorised outright: chromium is [Ar] 3d5 4s1 and copper is [Ar] 3d10 4s1, because exactly half-filled and fully filled d subshells gain exchange energy and symmetry stability. For cations, always remove electrons from 4s before 3d — Fe3+ is [Ar] 3d5, not [Ar] 3d3 4s2.

## What you must remember

- **(n + l) rule with tie-break:** lower (n + l) fills first; equal (n + l) means lower n wins — 4s (4) beats 3d (5), but 3d ties with 4p and takes the lower n.
- **Hund's rule payoff:** maximum unpaired electrons, parallel spins; the 3d5 4s1 of chromium has six unpaired electrons and exchange-energy stabilisation.
- **The two named exceptions:** Cr (Z 24) [Ar] 3d5 4s1 and Cu (Z 29) [Ar] 3d10 4s1; NCERT explains them by exactly half-filled and completely filled subshell stability.
- **Cation rule:** remove from the outermost shell first — 4s goes before 3d for transition metals, so Fe2+ is 3d6 and Fe3+ is 3d5 (extra stability of d5 again).
- **Anions just add:** Cl− is [Ne] 3s2 3p6, isoeelectronic with argon; O2− and Na+ also mirror neon-family shells.
- **Magnetic reading:** unpaired electrons give paramagnetism; count them from the ion's configuration — d5 Mn2+ has five unpaired spins against four in d6 Fe2+.
- **Capacity ceiling:** configurations respect 2n^2 per shell, and the outermost shell can never hold more than 8 electrons.

## Writing configurations the exam way

Build copper step by step to see why the exception exists. Straight Aufbau gives [Ar] 3d9 4s2 with one paired-and-one-single d arrangement; promoting one 4s electron yields [Ar] 3d10 4s1, a perfectly filled d subshell whose symmetric charge distribution and added exchange energy repay the small promotion cost. Chromium follows the same logic to 3d5, where five parallel spins maximise exchange interactions.

Now convert to ions, where NEET does its real business. Iron: [Ar] 3d6 4s2. For Fe2+, strip the two 4s electrons first (they sit outside the 3d cloud), leaving [Ar] 3d6 with four unpaired electrons; for Fe3+, one more d electron leaves [Ar] 3d5 — five unpaired spins and a half-filled subshell, which is precisely why Fe3+ is the more stable iron state in aqueous chemistry. A question flashing "Fe2+ = [Ar] 3d4 4s2" as an option is exploiting candidates who forget that 4s empties first. For lanthanoids the same caution applies with 6s and 4f/5d ordering.

## Where aspirants lose marks

The chapter's marks die in three places. First, the tie-break: candidates memorise "4s before 3d" but freeze when asked whether 4f or 5d fills after 6s — apply (n + l): 4f (7) beats 5d (7) on lower n, so lanthanoids fill 4f before the 5d transition row completes. Second, ion formation from 4s versus 3d, covered above — the physical reason is 4s electrons are the outermost and ionised first even though they filled first. Third, unpaired-electron counting for magnetic moment: μ = √(n(n + 2)) BM needs n from the ion's configuration, not the atom's, so Mn2+ (d5, 5.92 BM) is quoted against Cr3+ (d3, 3.87 BM) in options. Quote configurations in the NCERT order (3d before 4s) to avoid mark-sheet disputes in integer-type questions.

## Frequently asked questions

### Why is chromium's configuration 3d5 4s1 instead of 3d4 4s2?

The exactly half-filled 3d5 subshell gains exchange energy and symmetrical distribution stability that outweighs promoting one 4s electron.

### Which orbital fills first, 4s or 3d, and why?

4s fills first because its (n + l) sum 4 is lower than 3d's 5; but on ionisation, 4s electrons are removed before 3d.

### What does Hund's rule of maximum multiplicity state?

Electrons occupy degenerate orbitals singly with parallel spins before pairing begins, maximising total spin and lowering energy through exchange.

### What is the electronic configuration of Fe3+?

Iron ([Ar] 3d6 4s2) loses two 4s electrons then one 3d electron, giving [Ar] 3d5 — a half-filled, extra-stable arrangement with five unpaired electrons.

### Which species is isoelectronic with Na+?

Mg2+, F−, O2− and Ne all hold 10 electrons; NEET typically tests this by mixing radii trends with the electron count.