📘 Class 12 – Chemistry
Chapter 01: The Solid State ⚪
📌 PART – 3
Welcome back to ExamsPoint Notes, your trusted platform for JEE, NEET, and Board exam handwritten content! This is Part 3 of Class 12 Chemistry Chapter 1: The Solid State, where we explore important concepts like density, packing efficiency, and close packing of solids.
🔍 Topics Covered in This Video (Solid State – Part 3):
✅ Density of Unit Cell
The density of a crystalline solid is calculated using its unit cell properties. The formula used is:
Density (ρ) = (Z × M) / (a^3 × N_A)
Where:
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Z = Number of atoms per unit cell
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M = Molar mass
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a = Edge length of the unit cell
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N_A = Avogadro's number
By plugging values into this equation, students can determine the theoretical density of metals and other solids. This helps in comparing theoretical and experimental data and understanding void spaces in the lattice.
✅ Packing Efficiency / Packing Fraction
Packing efficiency refers to the percentage of total space occupied by particles in a unit cell. It highlights how tightly atoms are packed:
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Simple Cubic (SC): 52.4%
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Body-Centered Cubic (BCC): 68%
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Face-Centered Cubic (FCC)/Hexagonal Closed Packing (HCP): 74%
Higher efficiency means lesser void space, making the structure more stable. This concept is significant for comparing different types of unit cells in real-world applications.
✅ Percentage Packing
This is essentially the same as packing efficiency. It indicates how much volume in a crystal is actually occupied by particles, with the rest being empty space (voids). FCC structures, for example, have the highest packing percentage of 74%, which explains their presence in many metals.
✅ Closed Packed vs Loosely Packed Structures
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Closed Packed Structures have the highest packing efficiency and minimal voids. Examples include FCC and HCP arrangements.
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Loosely Packed Structures, like SC, have large voids and lower density.
This comparison is important in material science to predict properties like hardness, strength, and conductivity.
✅ Classification of Close-Packed Structures (CPS)
CPS are categorized based on dimensions:
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1D Close Packing: Atoms are arranged in a single line touching each other.
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2D Close Packing:
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Square Packing: Rows and columns aligned. Efficiency ~52%.
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Hexagonal Packing: Each sphere surrounded by six others. Efficiency ~60%.
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3D Close Packing:
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Hexagonal Close Packing (HCP): ABABAB... layer sequence.
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Cubic Close Packing (CCP/FCC): ABCABC... layer sequence.
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Understanding these arrangements explains why certain solids are more compact and stable than others.
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