# Crystal Lattice: Definition & Structure

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• 0:00 Definition of Crystal Lattice
• 1:25 Crystal Lattice Structure
• 2:18 Symmetry
• 2:55 Classification
• 3:43 Types of Crystal…
• 4:43 Lesson Summary

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Lesson Transcript
Instructor: Danielle Reid

Danielle has taught middle school science and has a doctorate degree in Environmental Health

Did you know crystal solids have a certain arrangement? Learn about this arrangement, called a crystal lattice, and explore their structures. Test yourself at the end of this lesson by taking a short quiz.

## What Is A Crystal Lattice?

Do you know what common table salt (NaCl) and a beautiful, shiny diamond have in common? I know what you're thinking - how on earth could the salt on your french fries have anything in common with the expensive diamonds found in jewelry? Well, based on their structure, they are both solid objects that contain tiny crystals interlocking together.

In chemistry, crystals are a type of solid material composed of atoms or groups of atoms that are arranged in a three-dimensional pattern that is very ordered. In a crystal, the groups of atoms are repetitive at evenly spaced intervals, all maintaining their orientation to one another. In other words, the geometric shape of a crystal is highly symmetrical. When you see the word 'symmetrical,' think about the perfect proportion and balance of these atoms in a crystal. Now that we know what a crystal is, and that is can be found inside our table salt and a sparkly diamond, let's look at crystal lattices.

A crystal lattice is the arrangement of these atoms, or groups of atoms, in a crystal. These atoms or groups of atoms are commonly referred to as points within a crystal lattice site. Thus, think of a crystal lattice site as containing a series of points arranged in a specific pattern with high symmetry. Note that these points don't tell you the position of an atom in a crystal. They are simply points 'in space' oriented in such a way to build a lattice structure.

## Crystal Lattice Structure

As we look at the structure of a crystal lattice, keep in mind the patterns of arrangement of each point as well as their symmetry. Crystal lattice sites are only viewed microscopically and are invisible to the naked eye. In order to view these structures, we must take a crystal (solid object), place it under a microscope, and view the crystal lattice sites.

The structure of a crystal lattice is shown here. Recall that a crystal lattice is the arrangement of atoms in a crystal (the black and white points in the image here are your atoms). This arrangement can be defined as the intersection of three parallel planes. So, if we slice this diagram into three parts you will see three different planes. When these planes intersect with one another, the result is a three-dimensional network that has faces. Think of each face as a box that is arranged in a parallel manner.

## Symmetry

You don't think I would forget about our friend symmetry, did you? Certainly not; in fact, these boxes (or faces) contribute to the symmetry of a crystal lattice structure. Each box contains the symmetry information required to ensure the crystal structure is translational.

Translational symmetry occurs when an object moves (or translates) at a certain distance in a certain location. For example, let's say you would like to build a patterned wood floor. You lay one style of wood down, diagonally, every 20 inches. This style of wood, in a particular pattern, has a certain distance (20 inches) and direction (diagonal).

## Classification

Crystal lattices can be classified as either monatomic or polyatomic. This classification is based on the kind of atoms present in the face within a lattice structure. If there is only one type of atom used to make a face (or box), this is monoatomic. A polyatomic crystal lattice contains more than one type of atom used to make a face.

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