Atomic Structure & Chemical Bonding: Models, Orbitals, Bonds & Properties

Core Concepts: The Atom’s Blueprint

1. The Quantum Atom: From Bohr to Schrödinger

Atoms are not tiny solar systems—electrons exist as probability clouds (orbitals) defined by quantum numbers. The Bohr model (circular orbits) was replaced by wavefunctions (ψ) that give electron density regions.

Quantum Numbers and Orbitals

Every electron in an atom is defined by four quantum numbers:

• Principal (n): Energy level (1, 2, 3…). Higher n → larger orbital → higher energy.
• Angular (ℓ): Shape of orbital (0 = s, 1 = p, 2 = d, 3 = f).
• Magnetic (mℓ): Orientation in space (–ℓ to +ℓ).
• Spin (ms): ±½ (Pauli exclusion principle: no two electrons share all four).

Orbital Shapes:

• s-orbitals: Spherical (1s, 2s, 3s…).
• p-orbitals: Dumbbell-shaped (px, py, pz).
• d-orbitals: Cloverleaf (dxy, dxz, dyz, dx²–y², dz²).
• f-orbitals: Complex (7 orientations).

Why it matters: Orbital overlap determines bond strength and molecular geometry.

Electron Configuration Rules

1. Aufbau Principle: Fill lowest-energy orbitals first (1s < 2s < 2p < 3s < 3p…).
2. Hund’s Rule: Fill degenerate orbitals (same energy) singly before pairing.
3. Pauli Exclusion Principle: Max 2 electrons per orbital (opposite spins).

Worked Example: Write the electron configuration of Fe (Z=26).

• Step 1: Fill in order: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶.
• Step 2: Use noble-gas shorthand: [Ar] 4s² 3d⁶.
• Step 3: Verify: 2 + 2 + 6 + 2 + 6 + 2 + 6 = 26.

Real-World Tie: Ncell’s 5G Base Stations use copper (Cu) and silver (Ag) for conductivity. Their electron configurations explain why:

• Cu: [Ar] 4s¹ 3d¹⁰ → loses 1 electron easily → good conductor.
• Ag: [Kr] 5s¹ 4d¹⁰ → similar mobility → even better conductor.

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2. Periodic Trends: Why Elements Behave Differently

The periodic table organizes elements by atomic number (Z) and reveals trends in:

• Atomic Radius: Decreases left→right (increasing nuclear charge pulls electrons closer), increases top→bottom (new shells added).
• Ionization Energy (IE): Energy to remove an electron. Highest at noble gases (He, Ne, Ar) → full valence shell.
• Electronegativity (EN): Ability to attract electrons. Fluorine (F) is most electronegative (EN=4.0).

Real-World Tie: eSewa’s QR Codes use silicon (Si) chips, where:

• SiO₂ (silica) has a tetrahedral structure (like CH₄) → rigid, heat-resistant.
• Doping (adding P/As) creates polar covalent bonds for conductivity.

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Bonding in Engineering Materials

methanemethanechloromethanechloromethane

5. Bonding in Solids: Why Materials Have Different Properties

Worked Example: Why Graphite Conducts Electricity (But Diamond Doesn’t)

• Graphite: Delocalized π-electrons (from sp² hybridized C) → conducts.
• Diamond: sp³ hybridized C → all electrons localized → insulator.

Real-World Tie: NEPSE’s Stock Market Data relies on fiber-optic cables made of:

• Silica (SiO₂): Covalent network → transparent, high-speed data transmission.
• Doped with Ge: Adds polar covalent bonds for signal amplification.

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Exam Tip: How to Score Full Marks

1. Diagrams > Words: Always draw Lewis structures, orbital shapes, or VSEPR geometries for bonding questions.
2. Quantum Numbers: For electron configurations, always list all four quantum numbers for the last electron.
3. Trend Questions: Use the periodic table trends (radius, IE, EN) to explain why a reaction occurs (e.g., "F₂ is more reactive than Cl₂ because EN decreases down Group 17").
4. Real-World Links: Connect bonding to materials (e.g., "Metallic bonding in Al makes it lightweight for aircraft").
5. Common Pitfalls:
   • Don’t confuse ionic vs. covalent (metal + nonmetal = ionic).
   • Remember lone pairs affect shape (e.g., H₂O is bent, not linear).
   • Hydrogen bonding is stronger than dipole-dipole but weaker than covalent bonds.

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Final Checklist Before Exam: ✅ Can you draw the orbital shapes for s, p, d? ✅ Do you know the VSEPR shapes for 2–6 electron pairs? ✅ Can you explain why NaCl has a high MP but CH₄ does not? ✅ Are you ready to apply bonding to real materials (e.g., batteries, wiring, packaging)?