The Types of Crystals
Crystals are formed by atoms, ions, or molecules. We can classify crystals into four types according to the kind of particles that make up the crystal and the forces that hold them together.
1. Ionic crystals – Positive and negative ions are held in the crystal arrangement by electrostatic attraction. Because these forces are strong, ionic substances have high melting points. Ionic crystals are hard and brittle. It shows what happens if an attempt is made to deform and ionic crystal. Because of the movement of one plane of ions over another, ions with the same charge are brought next to one another. The crystal breaks into fragments. Ionic compounds are good conductors of electricity when molten or in solution but not in the crystalline state, where the ions are not free to move.
2. Molecular crystals – Molecules occupy positions in crystals of covalent compounds. The intermolecular forces that hold the molecules in the crystal structure are not nearly as strong as the electrostatic forces that hold ionic crystals together. Molecular crystals, therefore, are soft and have low melting points, usually below 300 degrees Celsius.
London forces hold nonpolar molecules in the structure. In crystals of polar molecules, dipole-dipole forces as well as London forces occur. Polar compounds, therefore, generally melt at slightly higher temperatures than nonpolar compounds of comparable molecular size and shape.
In general, molecular substances do not conduct electricity in the solid or liquid states. A few molecular compounds, such as water, dissociate to a very slight extent and produce low concentrations of ions; these liquids are poor electrical conductors.
3. Network crystals – in these crystals, atoms occupy positions and they are joined by a network of covalent bonds. The entire crystal can be looked at as one giant molecule. In diamond, an example of this type of crystal, carbon atoms are bonded by covalent bonds into a three-dimensional structure. Materials of this type have high melting points and are extremely hard because of the large number of covalent bonds that would have to be broken to destroy the crystal structure. Network crystals do not conduct electricity.
4. Metallic crystals – the outer electrons of metal atoms are loosely held and move freely throughout a metallic crystal. The remainder of the metal atoms, positive ions occupies fixed positions in the crystal. The negative cloud of the freely moving electrons, sometimes called an electron gas or a sea of electrons, binds the crystal together. This binding force, called a metallic bond.
The metallic bond is strong. Most metals have high melting points, high densities and structures in which the positive ions are packed together closely (called closest-packed arrangements). Unlike with ionic crystals, the positions of the positive ions can be altered without destroying the crystal because of the uniform cloud of negative charge provided by the freely moving electrons. Most metallic crystals, therefore, are easily deformed, and most metals are malleable (capable of being beaten into shape) and ductile (capable of being drawn into wire). The freely moving electrons are also responsible for the fact that most metals are good conductors of electricity.
A crystal structure is a symmetrical array of atoms, ions, or molecules arranged in a repeating, three-dimensional pattern. The symmetry of a crystal can be described in terms of a crystal lattice. A lattice is a three-dimensional arrangement of points that represent sites with identical surroundings in the same orientation. If one starts with any lattice point and goes a definite distance in a set direction, a second lattice point will be encountered. The points are identical and have identical surroundings.
A crystal lattice can usually be derived from a crystal structure by replacing the centers of the material units (atoms, for example) with lattice points. Notice, however, that the definition of a crystal lattice requires that the lattice points and their surroundings be identical. A lattice of an ionic crystal, therefore, can be defined with the points centering on the cations, or on the anions, or on some sites between the two.
A crystal lattice can be divided into identical parts called unit cells. In theory, a lattice can be reproduced by stacking its unit cells in three dimensions. The idea of a unit cell can also be applied to a crystal structure. These units cells are considered to consist of all the material units of which the crystal is composed (both cations and anions of an ionic crystal) rather than just lattice points. Keep in mind that the unit cell of a crystal structure has all the components of the crystal in the correct ratios. Repeating the unit cell of a crystal structure in three dimensions generates the structure itself.
The simplest types of unit cells are the cubic unit cells. Notice that it is possible to have points at positions other than the corners of the unit cells. In the body-centered cubic unit cell, a point occurs in the center of the cell. In the face-centered cubic unit cell, a point occurs in the center of each face of the cell.
IN crystals of metals, we may assume that atoms occupy lattice positions (even though the outer electrons of the metal atoms move freely throughout the structure). In counting the number of atoms per unit cell, one must keep in mind that atoms on corners or faces are shared by adjoining cells. Eight unit cells share each corner atom, and two unit cells share each face-centered atom. A body-centered atom is not shared between unit cells and belongs exclusively to the unit cell in which it is found.