What is Electron Configuration?
Electron configuration describes the arrangement of electrons in an atom's orbitals and subshells. It follows the Aufbau principle (building-up rule): electrons fill orbitals of lowest energy first, moving to higher-energy orbitals only when lower ones are full.
Electron configuration is the distribution of electrons across orbitals and subshells in an atom. It follows the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p... up to the total atomic number.
- 1↓Hydrogen (H, Z=1)1s¹ — 1 electron in 1s orbital
- 2↓Helium (He, Z=2)1s² — 2 electrons, 1s orbital is full
- 3↓Lithium (Li, Z=3)1s² 2s¹ — third electron enters 2s subshell
- 4↓Carbon (C, Z=6)1s² 2s² 2p² — filling 2p orbitals (Hund's rule)
- 5Oxygen (O, Z=8)1s² 2s² 2p⁴ — 4 electrons now in 2p orbitals
Step-by-step worked examples
Write the electron configuration for nitrogen (N, Z=7).
Follow the Aufbau order: 1s² 2s² 2p³ Nitrogen has 7 electrons total Configuration: 1s² 2s² 2p³
What does the exponent in 2p⁵ mean?
The exponent tells how many electrons occupy that subshell 2p⁵ means 5 electrons in the 2p subshell Example: Chlorine (Cl) ends in 2p⁵
Draw the orbital diagram for beryllium (Be, Z=4).
Be configuration: 1s² 2s² 1s: [↑↓] 2s: [↑↓] Both 1s and 2s subshells are full
Flashcards
Quick quiz
Q1.What is the electron configuration of oxygen (O, Z=8)?
Q2.How many electrons can fit in a p subshell?
Q3.Which subshell fills after 3p?
Q4.Write the electron configuration of argon (Ar, Z=18).
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Common mistakes
Filling 3d before 4s. — Correct: After 3p, 4s fills next; 3d fills after 4s (due to orbital energies).
Ignoring Hund's rule in p, d, f subshells. — Correct: Electrons fill orbitals singly first (parallel spins), pairing only when needed.
Confusing electron configuration with valence electrons. — Correct: Configuration shows all electrons; valence = outermost shell only.
Writing 2p⁷ (more than 6). — Correct: p subshell max is 6 electrons (3 orbitals × 2 each).
FAQ
What is electron configuration and why does it matter?
Electron configuration describes how electrons arrange in orbitals. It predicts chemical properties, reactivity, and bonding behaviour.
What is the Aufbau principle?
Electrons fill orbitals starting with the lowest-energy orbitals (1s) and moving to higher ones only when lower ones are full.
How do you write electron configuration in shorthand?
Use noble-gas notation: [Ne] 3s² 3p⁴ for sulfur, where [Ne] represents the 10-electron neon core.
What is an orbital diagram?
A visual representation using boxes for orbitals and arrows (↑↓) for electrons, showing which orbitals are occupied and electron spins.