What Is the Atomic Structure of a Crystal?

Solids are divided into two categories: crystalline solids and amorphous solids. A crystalline solid has a long-range order that is rigid. Examples include table salt and ice. Amorphous solids include rubber, plastic, and even crystal glass, which is a misnomer.

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

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%.

References

• Chemistry; Raymond Chang; 1984
• Stein College: Glass Glossary
• Purdue U.: Unit Cell

Resources

• Stein College: Glass Glossary
• Purdue U.: Unit Cell