Chemistry Bridge Course - Lecture 8 | CHEMCA JEE & NEET

CHEMCA

JEE & NEET Academy by Abhishek Sengar

Lecture 8 Atomic Structure Target: Class 10 to 11 Transition (JEE/NEET)

Electronic Configuration of Atoms

Welcome to Lecture 8 of the CHEMCA Bridge Course! Electronic configuration is the cornerstone of both Inorganic and Organic Chemistry. In this session, Abhishek Sengar Sir moves beyond basic $2,8,8$ shells into subshells ($s, p, d, f$) and orbital configurations using the Aufbau Principle, with a special focus on half-filled/fully-filled exceptions.

Video Lecture Broadcast

Instructor: Abhishek Sengar Sir Published: April 20, 2026 Subject: Electronic Configuration

Interactive Lecture Timestamps

Click any topic to skip the video directly to that specific concept explanation.

In-Depth Lecture Notes & Summary

Abhishek Sir's Housing Analogy

Quantum mechanical descriptions of electrons can be highly abstract. To simplify, Abhishek Sengar Sir introduces a intuitive housing analogy to visualize subatomic arrangements:

Shell / Orbit The Building

K, L, M, N... ($n = 1, 2, 3, 4$)

Subshell The Flat

$s, p, d, f$ ($l = 0, 1, 2, 3$)

Orbital The Room

Contains atomic spatial orbitals

Electron The Resident

Max 2 per room with opposite spins

Subshell & Orbital Capacities

Each subshell ($s, p, d, f$) possesses a specific number of constituent orbitals. According to Pauli's Exclusion Principle, each orbital accommodates a maximum of 2 electrons with opposite spins:

Subshell	Quantum ($l$)	Number of Orbitals	Maximum Electrons
$s$ (sharp)	$l = 0$	1	2
$p$ (principal)	$l = 1$	3	6
$d$ (diffuse)	$l = 2$	5	10
$f$ (fundamental)	$l = 3$	7	14

The Aufbau Principle & filling rules

The German word Aufbau translates to "building up". The principle states:

"In the ground state of an atom or ion, electrons fill atomic orbitals of the lowest available energy levels before occupying higher levels."

The standard energy filling sequence is determined by the $(n+l)$ rule:

$$1s \to 2s \to 2p \to 3s \to 3p \to 4s \to 3d \to 4p \to 5s \dots$$

The $4s$ vs. $3d$ Mystery: Notice that the $4s$ subshell is filled before the $3d$ subshell. This is because the $4s$ orbital possesses lower energy than the $3d$ orbital.

Stability Anomalies: Chromium & Copper

Among the first 30 elements, two show highly critical exceptions due to the **thermodynamic stability of half-filled and fully-filled subshells**:

Chromium ($Z = 24$)

• Expected: $[Ar]\,4s^2 3d^4$
• Actual: $[Ar]\,4s^1 3d^5$
Reason: Symmetrical distribution and maximum exchange energy of a half-filled $3d^5$ subshell makes it highly stable.

Copper ($Z = 29$)

• Expected: $[Ar]\,4s^2 3d^9$
• Actual: $[Ar]\,4s^1 3d^{10}$
Reason: The extra stability associated with a completely fully-filled $3d^{10}$ subshell drives this transfer.

Aufbau Config Solver & Visualizer

Input any Atomic Number ($Z = 1$ to $30$) or select a Nobel Gas to generate the orbital configuration and outer shell box diagram live!

Select Preset Element:

Atomic Number (Z)

Analysis for element: Chromium (Cr)

Aufbau Configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵

Shell-wise Configuration (K, L, M, N): 2, 8, 13, 1

Outer Subshell Box Diagrams (Valence)

Lecture 8 Concept Test

Validate your understanding of subshells, Aufbau filling rules, and orbital capacities.

Question 1 of 5

Score: 0/0

Stuck on Quantum Configurations?

If you have doubts regarding why the $4s$ subshell fills before $3d$, or why chromium and copper undergo spin rearrangement, email Abhishek Sengar Sir directly!

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Chemistry Bridge Course Lecture 8