As we mentioned above, the molecules that make up solids
are generally held together by ionic or strong covalent bonding,
and the attractive forces between the atoms, ions, or molecules
in solids are very strong. In fact, these forces are so strong that
particles in a solid are held in fixed positions and have very little
freedom of movement. Solids have definite shapes and definite volumes
and are not compressible to any extent. There are a few types of
solids that you should be familiar with for the SAT II Chemistry
test, and we’ve listed them below. However, we will start by saying
that there are two main categories of solids—crystalline solids
and amorphous solids. Crystalline solids are those
in which the atoms, ions, or molecules that make up the solid exist
in a regular, well-defined arrangement. The smallest repeating pattern
of crystalline solids is known as the unit cell, and
unit cells are like bricks in a wall—they are all identical and
repeating. The other main type of solids are called the amorphous
solids. Amorphous solids do not have much order in
their structures. Though their molecules are close together and
have little freedom to move, they are not arranged in a regular
order as are those in crystalline solids. Common examples of this
type of solid are glass and plastics.
There are four types of crystalline solids, all of which
you should be familiar with for the exam.
Ionic solids—Made up of positive and negative
ions and held together by electrostatic attractions. They’re characterized
by very high melting points and brittleness and are poor conductors
in the solid state. An example of an ionic solid is table salt,
Molecular solids—Made up of atoms or molecules
held together by London dispersion forces, dipole-dipole forces,
or hydrogen bonds. Characterized by low melting points and flexibility
and are poor conductors. An example of a molecular solid is sucrose.
Covalent-network (also called atomic) solids—Made
up of atoms connected by covalent bonds; the intermolecular forces
are covalent bonds as well. Characterized as being very hard with
very high melting points and being poor conductors. Examples of
this type of solid are diamond and graphite, and the fullerenes.
As you can see below, graphite has only 2-D hexagonal structure
and therefore is not hard like diamond. The sheets of graphite are held
together by only weak London forces!
Metallic solids—Made up of metal atoms that
are held together by metallic bonds. Characterized by high melting
points, can range from soft and malleable to very hard, and are good
conductors of electricity.