1. Chapter 13.3
The Nature of Solids

2. A Model for Solids
Solids generally have an orderly arrangement of particles in a fixed location. The atoms, ions, or molecules of a solid usually are packed tightly together. Solids are usually dense and not easy to compress. They also do not flow.
When you heat a solid their particles vibrate and the kinetic energy increases. When the organization of particles in a solid breaks down the solid melts. The melting point is the temperature the solid changes to a liquid. The melting point and freezing point of a solid are the same temperature. At that temperature the liquid and solid phases are in equilibrium.

4. Crystal Structure and Unit Cells
Most solids are crystalline. A crystal has particles arranged in an orderly, repeating, three-dimensional pattern called a crystal lattice.
The shape of a crystal shows the arrangement of the particles in the solid. The type of bonding between particles in crystals will determine their melting points. Usually ionic solids have high melting points, while molecular solids have low melting points. Not all solids can melt - think of wood or cane sugar.

6. Crystal Systems
A crystal has sides, or faces. There are 7 groups of crystals (or crystal systems). The shapes differ based on the angles between the faces and the number of edges of equal length on each face.

8. Allotropes
Some solids can exist in more than one form. For example carbon - it can exist as diamond or it can exist as graphite. The arrangement of carbon atoms are different in these forms.
Allotropes are two or more different molecular forms of the same element in the same physical state. They will be made of the same atoms but because of different arrangement they have different characteristics.
Other solids that have allotropes are phosphorus, sulfur, oxygen, boron, and antimony.

10. Non-Crystalline Solids
Some solids are amorphous. Amorphous solids do not have an ordered internal structure. Examples are rubber, plastic, and asphalt. Their particles are arranged randomly.
Glass is also amorphous. Some glasses are supercooled liquids. That means the internal structure is between a crystalline solid and a free-flowing liquid. Glasses do not have a definite melting point, instead they soften.