As we mentioned above, the molecules that make up solids are generallyheld together by ionic or strong covalent bonding, and the attractiveforces between the atoms, ions, or molecules in solids are very strong.In fact, these forces are so strong that particles in a solid are heldin fixed positions and have very little freedom of movement. Solidshave definite shapes and definite volumes and are not compressible toany extent. There are a few types of solids that you should be familiarwith for the
SAT II Chemistry test, and we’ve listed them below.However, we will start by saying that there are two main categories ofsolids—crystalline solids and amorphous solids.
Crystalline solidsare those in which the atoms, ions, or molecules that make up the solidexist in a regular, well-defined arrangement. The smallest repeatingpattern of crystalline solids is known as the
unit cell, andunit cells are like bricks in a wall—they are all identical andrepeating. The other main type of solids are called the amorphoussolids.
Amorphous solids do not have much order in theirstructures. Though their molecules are close together and have littlefreedom to move, they are not arranged in a regular order as are thosein crystalline solids. Common examples of this type of solid are glassand plastics.
There are four types of crystalline solids, all of which you should be familiar with for the exam.
Ionic solids—Made up of positiveand negative ions and held together by electrostatic attractions.They’re characterized by very high melting points and brittleness andare poor conductors in the solid state. An example of an ionic solid istable salt, NaCl.
Molecular solids—Made upof atoms or molecules held together by London dispersion forces,dipole-dipole forces, or hydrogen bonds. Characterized by low meltingpoints and flexibility and are poor conductors. An example of amolecular solid is sucrose.
Covalent-network (also called atomic) solids—Madeup of atoms connected by covalent bonds; the intermolecular forces arecovalent bonds as well. Characterized as being very hard with very highmelting points and being poor conductors. Examples of this type ofsolid are diamond and graphite, and the fullerenes. As you can seebelow, graphite has only 2-D hexagonal structure and therefore is nothard like diamond. The sheets of graphite are held together by onlyweak London forces!
.{ }D7S wG~
Metallic solids—Made up ofmetal atoms that are held together by metallic bonds. Characterized byhigh melting points, can range from soft and malleable to very hard,and are good conductors of electricity.
