AP Chemistry · Unit 2 Molecular & Ionic Compound Structure
Learning Objectives
Unit Cell: The smallest repeating 3D building block that tiles space to build the entire crystal. All lattice geometry comes from a single box.
Packing Efficiency: What percentage of the unit cell volume is actually occupied by atoms vs empty void? FCC/HCP reach the theoretical maximum of 74%.
Coordination Number: How many nearest neighbors touch each atom. Higher CN = denser packing = typically higher melting point.
Atoms Per Unit Cell: Corner atoms are shared by 8 cells (×⅛), edges by 4 (×¼), faces by 2 (×½), body center by 1 (×1).
Tags
CrystalFCCBCCHCPPacking
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Crystal Structure
Atom Radius = 40%
UNIT CELL PROPERTIES
Atoms per Cell:1
Coordination #:6
Packing Efficiency:52.4%
Edge Relationship:a = 2r
Simple Cubic: Each corner atom is shared among 8 unit cells, so only 1/8 of each actually belongs here. 8 corners × 1/8 = 1 atom per cell. The least efficient packing (52.4%) — only Polonium uses it naturally.