Crystals

CRYSTALS

Crystals or solid forms of substances will have “faces”. These will have certain geometrical relationships, resulting in symmetry in crystals. Different solid molecules will have different crystalline structures. Sodium chloride is an easy one because if forms a cubic structure that can be seen with a simple magnifying glass. Figure 37 shows what a cubic crystal looks like molecularly:

Figure 37.

Irregularities or differences in the pattern of crystal shapes are referred to as “crystal habits”. With sodium chloride, the general shape of things is that it grows into a completely cubic shape. Partial melting and impurities can create different habits of the cubic shape. While a crystal can become broken or distorted, it will have the same angles between its corresponding faces. This is referred to as the law of constant angles.

When a crystal is cleaved, it will cleave along particular cleavage planes according to the law of constant angles. The cleavage angles are determined by the molecular structure of the molecules in solid form. The different possible ordered shapes include the cube, the rectangle, the parallelogram, the rhombus, and the hexagonal shape. The rhomboid shape is similar to the hexagonal shape, with the rhomboid shape being simpler. These are the two-dimensional shapes that crystals can take.

As you know, snowflakes take on a hexagonal shape. Why is this the case? It’s because water crystals or ice forms a hexagonal pattern when ice crystalizes. They form unique

shapes but, in reality, snowflakes are entirely based on the hexagon. Figure 38 shows the hexagonal basis of the snowflake or ice crystal:

Figure 38.

In reality, there are fourteen different lattice shapes, which are referred to as Bravais lattices. These shapes or “crystal systems” can be defined by the length of the sides of the 3-dimensional shapes and the angles that exist between the sides.

The different crystal systems in solid elements and molecules include the following:

• Cubic—there are three possible cubic shapes that can be made molecularly.

• Tetragonal—these involve two equal sides and one unequal side. There are two possible molecular ways to make this shape.

• Orthorhombic—there are four possible molecular ways to make this. The angles are all 90 degrees but the side length are different.

• Hexagonal—there is one way to create this shape. It is created from three rhombic prisms.

• Trigonal (rhombohedral)—there is one way to make this shape molecularly. The sides are all the same length but one of the angles is not 90 degrees.

• Monoclinic—this is a three-dimensional shape in which none of the sides are equal.

• Triclinic—this is a shape in which no side is equal and no angle is 90 degrees.

Figure 39 shows what the different shapes of crystals look like:

Figure 39.

There are four types of crystals as described by the particles that make them up. These include: 1) ionic crystals, 2) metallic crystals, 3) covalent network crystals, and 4) molecular crystals. Ionic crystals include things like sodium chloride (NaCl) and calcium fluoride (CaF2). Metallic crystals include mercury (Hg) and sodium (Na). Covalent network crystals can be things like diamonds (carbon) and silicon oxide (SiO2). Molecular crystals can include iodine (I2) and ammonia (NH3).

Ionic crystals will have alternating positively-charged cations and negatively-charged anions. These can be polyatomic or monoatomic ions. These are typical of group 1 or group 2 metals along with group 16 and group 17 nonmetals or polyatomic ions. They have high melting points and are extremely brittle. They do not conduct electricity as solids but do conduct electricity when in aqueous solutions. Figure 40 shows an ionic crystal from a molecular standpoint:

Figure 40.

Metallic crystals consist of a grouping of metal cations in a soup of valence electrons. These are good conductors of electricity because of the delocalized electrons in the group that do not belong to any particular nucleus. These have a wide range of melting points.

Covalent network crystals consist of atoms that are covalently bonded to their nearest neighbor. These include things like diamonds, quartz, many metalloids, and certain metal oxides. These have very high melting and boiling points and are extremely hard solids. They do not conduct electricity.