Representation

• Spheres and sticks represent atoms and repeating molecules in crystals. The center of each of their positions is called a "lattice point."
  
  

Unit Cells

• The smallest repeating structure in a crystal is called a "unit cell." There are seven types, depending on how many of the unit cell's sides are equal and how many of the sides' angles are right angles.
  
  

Centering

• Face-centered and body-centered lattices (DrBob, Daniel Mayer/Wikipedia)
  
  A sphere can be centered in the unit cell. For example, in a cubic cell, a sphere (lattice point) may sit in the center (called "body-centered") or six spheres may sit in the center of the six faces (called "face-centered").
  
  

Forces

• The forces responsible for the stability of a crystal run the full range, from covalent and ionic (which bind molecules) to Van der Waals forces. For example, in ice the H2O molecules are held together by Van der Waals forces.
  
  

Packing Efficiency

• Packing efficiency is the ratio of occupied to unoccupied space in a unit cell. For example, NaCl's cubic shape has a packing efficiency of 52%, but zinc's stabler hexagonal shape has packing efficiency of 74%.