Electronic Configuration
Understanding how electrons are arranged in atoms
Electronic configuration describes the arrangement of electrons in an atom. It determines the chemical properties of elements and their position in the Periodic Table.
1. Energy Levels and Orbitals
Main Energy Levels
Electrons occupy specific shells (energy levels) designated by principal quantum numbers n = 1, 2, 3...
Maximum electrons per level = 2n²
Atomic Orbitals
s-orbital
Spherical shape
Max: 2 electrons
p-orbital
Dumbbell shape (pₓ, pᵧ, p₂)
Max: 6 electrons total
2. The Three Governing Rules
Electrons fill the lowest energy orbitals first.
1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p...
An orbital can hold a maximum of two electrons with opposite spins.
Electrons fill empty orbitals of the same energy singly first, with parallel spins.
Hund's Rule Demonstration
Click to add electrons to the 2p orbitals (follows Hund's Rule):
3. Representing Electron Arrangement
sp Notation
Writing configuration as a string:
Nitrogen (Z=7): 1s² 2s² 2p³
Orbital Diagrams
Using boxes and arrows:
Nitrogen orbital diagram
4. Valence and Core Electrons
Electrons in the outermost energy level
Determine chemical reactivity and bonding
Electrons in filled inner levels
Do not participate in bonding
Sodium (Na) - Atomic number 11
Full configuration: 1s² 2s² 2p⁶ 3s¹
Core: 1s² 2s² 2p⁶ (10 electrons)
Valence: 3s¹ (1 electron)
5. Transition to the Periodic Table
The position of an element on the Periodic Table relates directly to its electron configuration:
Group 1
Li: 2s¹
Na: 3s¹
K: 4s¹
Group 2
Be: 2s²
Mg: 3s²
Ca: 4s²
Group 17
F: 2s² 2p⁵
Cl: 3s² 3p⁵
Group 18
Ne: 2s² 2p⁶
Ar: 3s² 3p⁶
Electronic Configuration Builder
Build Electron Configurations
Configurations for First 20 Elements
| Element | Symbol | Atomic Number | Electron Configuration |
|---|---|---|---|
| Hydrogen | H | 1 | 1s¹ |
| Helium | He | 2 | 1s² |
| Lithium | Li | 3 | 1s² 2s¹ |
| Beryllium | Be | 4 | 1s² 2s² |
| Boron | B | 5 | 1s² 2s² 2p¹ |
| Carbon | C | 6 | 1s² 2s² 2p² |
| Nitrogen | N | 7 | 1s² 2s² 2p³ |
| Oxygen | O | 8 | 1s² 2s² 2p⁴ |
| Fluorine | F | 9 | 1s² 2s² 2p⁵ |
| Neon | Ne | 10 | 1s² 2s² 2p⁶ |
| Sodium | Na | 11 | 1s² 2s² 2p⁶ 3s¹ |
| Magnesium | Mg | 12 | 1s² 2s² 2p⁶ 3s² |
| Aluminum | Al | 13 | 1s² 2s² 2p⁶ 3s² 3p¹ |
| Silicon | Si | 14 | 1s² 2s² 2p⁶ 3s² 3p² |
| Phosphorus | P | 15 | 1s² 2s² 2p⁶ 3s² 3p³ |
| Sulfur | S | 16 | 1s² 2s² 2p⁶ 3s² 3p⁴ |
| Chlorine | Cl | 17 | 1s² 2s² 2p⁶ 3s² 3p⁵ |
| Argon | Ar | 18 | 1s² 2s² 2p⁶ 3s² 3p⁶ |
| Potassium | K | 19 | 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ |
| Calcium | Ca | 20 | 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² |
Test Your Understanding
1. What is the electron configuration of Oxygen (atomic number 8)?
2. How many valence electrons does Magnesium (atomic number 12) have?
3. Which rule states that electrons fill empty orbitals singly before pairing up?
4. What is the maximum number of electrons that can occupy the 2p sublevel?
Key Terms
Key Takeaways
- Energy levels: n=1,2,3... hold max 2n² electrons
- Orbitals: s (spherical, 2 e⁻), p (dumbbell, 6 e⁻ total)
- Aufbau: Fill lowest energy orbitals first
- Pauli: Max 2 electrons per orbital, opposite spins
- Hund: Fill empty orbitals singly before pairing
- sp notation: 1s² 2s² 2p⁶...
- Valence electrons: Outermost shell - determine chemical properties
- Periodic Table connection: Group 1 = ns¹, Group 2 = ns², Group 17 = ns² np⁵
