TOPIC 2.2.5 • CORE CONTENT
Electronic Configuration
The Invisible Architecture: Mapping Electrons in Atomic Shells
🌌 The Electron Shell System
📚 The IGCSE Rules
1
Shell 1 (Nearest): Holds maximum 2 electrons
2
Shell 2: Holds maximum 8 electrons
3
Shell 3: Holds maximum 8 electrons (for elements 1-20)
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Filling Order: Always fill the lowest available shell first!
🏗️ Analogy: Hotel Floors
"Think of an atom as a hotel where electrons are guests checking in..."
Ground Floor (Shell 1): Tiny room - only 2 guests allowed
First Floor (Shell 2): Bigger - fits 8 guests
Second Floor (Shell 3): Also fits 8 guests
Rule: Guests MUST fill lower floors before moving to higher ones!
✏️ Notation Format
Sodium (11p)
2,8,1
2 + 8 + 1 = 11 electrons
Chlorine (17p)
2,8,7
2 + 8 + 7 = 17 electrons
Calcium (20p)
2,8,8,2
2 + 8 + 8 + 2 = 20 electrons
🎯 Cambridge IGCSE Key Points
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Shell Capacity: Shell 1 holds 2, Shells 2 & 3 hold 8 (for elements 1-20)
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Filling Order: Always fill lowest energy shell first
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Notation: Written as comma-separated numbers (e.g., 2,8,1)
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Noble Gases: Group VIII has full outer shells = unreactive
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Group Number: Equals number of outer shell electrons (Groups I-VII)
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Period Number: Equals number of occupied electron shells
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