Module 4: Electronic Configuration

📘 Module 4: Electronic Configuration

Targeted at beginners like Suharshi, with easy-to-follow explanations, examples, and visual imagination.

🧪 Module 4: Electronic Configuration – How Electrons Are Arranged in an Atom

🎯 Learning Goals:

• Understand the Bohr model of the atom.

• Learn how electrons are arranged in shells (K, L, M, N).

• Learn to write the electronic configuration of elements up to atomic number 20.

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🔹 1. Why Learn Electronic Configuration?

Every element behaves a certain way in chemical reactions. That behavior depends on how electrons are arranged in its atom.

📌 The electronic configuration tells us:

• How many electrons are there

• How they are spread across different energy levels (orbits/shells)

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🔹 2. The Bohr Model (Simplified Explanation)

Danish scientist Niels Bohr proposed that:

• Electrons revolve around the nucleus in fixed paths called shells or energy levels.

• Each shell can hold a fixed maximum number of electrons.

• These shells are called K, L, M, N (starting from the one nearest the nucleus).

💡 Think of it like planets orbiting the sun, where each planet can only move in a specific orbit.

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🔹 3. Shell Names and Their Maximum Electron Capacities

Each shell can hold a limited number of electrons. This is determined by the formula:
Maximum electrons = 2n²
(where n is the shell number: K=1, L=2, etc.)

Shell	Symbol	n	Max Electrons (2n²)
1st	K	1	2
2nd	L	2	8
3rd	M	3	18 (but we use 8 for the first 20 elements)
4th	N	4	32 (but rarely used below element 20)

🧠 For elements with atomic number ≤ 20, we follow the simplified pattern:

• Fill K shell first (max 2)

• Then L (max 8)

• Then M (max 8 for first 20 elements)

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🔹 4. How to Write Electronic Configuration (Step-by-Step)

Step 1: Find atomic number (number of electrons in neutral atom).
Step 2: Distribute the electrons in shells in the order: K → L → M → N
Step 3: Follow the capacity rule: K = 2, L = 8, M = 8

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🧪 Examples: Electronic Configuration of First 20 Elements

Element	Atomic Number	Electronic Configuration	Shellwise (K, L, M, N)
Hydrogen	1	1	1
Helium	2	2	2
Lithium	3	2, 1	2 (K), 1 (L)
Beryllium	4	2, 2	2 (K), 2 (L)
Boron	5	2, 3
Carbon	6	2, 4
Nitrogen	7	2, 5
Oxygen	8	2, 6
Fluorine	9	2, 7
Neon	10	2, 8
Sodium	11	2, 8, 1	K=2, L=8, M=1
Magnesium	12	2, 8, 2
Aluminium	13	2, 8, 3
Silicon	14	2, 8, 4
Phosphorus	15	2, 8, 5
Sulfur	16	2, 8, 6
Chlorine	17	2, 8, 7
Argon	18	2, 8, 8
Potassium	19	2, 8, 8, 1	K=2, L=8, M=8, N=1
Calcium	20	2, 8, 8, 2

💡 Note:

• Sodium (11) = 2, 8, 1

• Chlorine (17) = 2, 8, 7

• Argon (18) = 2, 8, 8 → a noble gas (full outer shell)

• Potassium (19) = 2, 8, 8, 1 → next shell (N) starts

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🔹 5. Special Terms to Know

• Valence shell: The outermost shell where electrons are present.

• Valence electrons: Electrons present in the valence shell.

• Stability: Atoms with full outermost shells (like helium or neon) are stable and don’t react easily.

🧠 That’s why noble gases (like Helium, Neon, Argon) are called inert gases.

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🎯 Practice Activities

🧩 Activity 1: Fill-in-the-Blank Table

Element	Atomic Number	K	L	M	N
Nitrogen	7
Magnesium	12
Potassium	19

✏️ Task: Fill the number of electrons in each shell.

🧠 Activity 2: Draw the Atom

Task: Draw Bohr diagrams of the following atoms with labeled shells and electrons:

• Sodium (11)

• Chlorine (17)

• Oxygen (8)

🎮 Activity 3: “Guess Who?”

Clues:

• I have 8 protons and 8 electrons. My electronic configuration is 2,6. Who am I?

• My configuration is 2,8,8. I don’t react with anyone. Who am I?

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📌 Summary

• Bohr model explains electron arrangement in shells.

• Shells are named K, L, M, N.

• Electrons fill from lower to higher shells based on capacity (2n² rule).

• Electronic configuration shows how electrons are arranged.

• Knowing configurations helps understand valency and chemical behavior.

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