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Electron Configuration

How electrons are arranged in atoms - orbitals, shells, and hybridization

What is Electron Configuration?

Electron configuration describes how electrons are arranged in an atom. Electrons fill orbitals in order of increasing energy (Aufbau principle). This arrangement determines atomic size, reactivity, bonding behavior, and ALL chemical properties. Understanding electron configuration is KEY to understanding chemistry!

🔬 Core Concepts

1. Shells (n levels)

Energy levels from nucleus outward: n=1 (2 e⁻), n=2 (8 e⁻), n=3 (18 e⁻), n=4 (32 e⁻)

Example: Li: 1s² 2s¹ has electrons in n=1 and n=2 shells

2. Subshells (s, p, d, f)

Within each shell: s (2), p (6), d (10), f (14) maximum electrons

Example: Carbon: 1s² 2s² 2p² fills s, s, then p orbitals

3. Orbitals

Regions of probability where electrons "live". s (1 orbital), p (3), d (5), f (7)

Example: 2p³ means 3 electrons in three p orbitals with parallel spins

4. Valence Electrons

Outermost electrons involved in bonding. Determine chemical properties!

Example: Group 17 halogens have 7 valence electrons. Need 1 more for stability!

5. Blocks (s, p, d, f)

Periodic table organized by which subshell is filling

Example: d-block = transition metals (filling d orbitals)

6. Hybridization

Mixing of atomic orbitals to form bonds (sp, sp², sp³, dsp², etc.)

Example: C in CH₄ is sp³ hybridized (4 bonds, tetrahedral shape)
Aufbau Principle: Electrons fill orbitals in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s...

📋 The Periodic Table as Orbital Chart

Understanding the periodic table:

■ s-block (Groups 1-2): Filling s orbitals (alkali metals, alkaline earth)

■ p-block (Groups 13-18): Filling p orbitals (main group nonmetals, halogens, noble gases)

■ d-block (Groups 3-12): Filling d orbitals (transition metals)

■ f-block (lanthanides/actinides): Filling f orbitals (inner transition metals)

Element Position Reveals Configuration! A element's position in the periodic table tells you what orbitals are being filled.

📊 Examples by Period

Element Atomic # Full Configuration Condensed Form Valence Electrons
Hydrogen (H) 1 1s¹ 1s¹ 1
Carbon (C) 6 1s² 2s² 2p² [He] 2s² 2p² 4
Oxygen (O) 8 1s² 2s² 2p⁴ [He] 2s² 2p⁴ 6
Iron (Fe) 26 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s² [Ar] 3d⁶ 4s² 8 (3d⁶ 4s²)
Iodine (I) 53 Long! (ends in 4d¹⁰ 5s² 5p⁵) [Kr] 4d¹⁰ 5s² 5p⁵ 7

🔗 Hybridization in Bonding

Hybridization Orbitals Mixed Geometry Example
sp 1s + 1p Linear (180°) BeH₂, CO₂
sp² 1s + 2p Trigonal Planar (120°) BH₃, C₂H₄ (alkenes)
sp³ 1s + 3p Tetrahedral (109.5°) CH₄, H₂O, NH₃
dsp² 1d + 1s + 2p Square Planar [Ni(CN)₄]²⁻
d²sp³ 2d + 1s + 3p Octahedral [Fe(CN)₆]³⁻
Key Insight: Hybridization determines molecular geometry, which determines molecular properties (polarity, reactivity, etc.)

🔬 Why This Matters