Electronic Configuration

The electronic configuration of an atom describes the arrangement of electrons in its energy levels (shells) around the nucleus.

Rules for Writing Electronic Configuration

1. Aufbau Principle:

   • Electrons fill lower energy levels first, before moving to higher levels.

   • Order of filling:

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

2. Pauli Exclusion Principle:

   • An orbital can hold a maximum of two electrons with opposite spins.

3. Hund’s Rule:

   • Electrons occupy degenerate orbitals (same energy) singly first, then pair up.

4. Maximum Electron Capacity of Shells (Based on 2n² rule):

   • K shell (n=1) → max 2 electrons

   • L shell (n=2) → max 8 electrons

   • M shell (n=3) → max 18 electrons

   • N shell (n=4) → max 32 electrons

Electronic Configuration of First 20 Elements (Hydrogen to Calcium)

Diagram: Electronic Configuration of First 20 Elements

(A visual representation of atomic structures showing electron distribution in shells from Hydrogen to Calcium.)