The Nature of Solids

By Allen Tang, Dan Jacobson, and Ted Dennis.

A Model for Solids

• Properties of solids reflect the arrangement of their particles

• Melting point-the temperature at which a solid changes into a liquid– Ionic compounds have high melting points– Molecular solids have relatively low melting points

Crystal Structure

• Crystal-the particles are arranged in an orderly, repeating, three-dimensional pattern called a crystal lattice

• Shape of a crystal reflect the arrangement of the particles

OBJECTIVE

17. What makes solids different from liquids and gases according to KMT?

• In solids, the particles are arranged in fixed locations.

OBJECTIVE18. Compare and contrast the terms crystal system and unit cell.

Crystal Systems

• A crystal has sides, or faces• Crystals are classified into seven distinct

crystal groups, or crystal systems• The angles at which the faces of a crystal

intersect are always the same for a given substance. – Thus, the crystal system of a sample of galena (a

cubic system) will be identical in shape to the crystal system of another similarly sized sample.

Unit Cell

• A unit cell is the smallest group of particles within a crystal that retains the geometric shape of the crystal.

• A crystal lattice forms from a repeating array of any one of the fourteen kinds of unit cells.

• A unit cell is similar to a crystal system in that it defines the shape of said system by the bonding of its atoms, yet it is different in that a crystal system can consist of any amount of compounds while still retaining the structure of its original system, while a unit cell exists only as the smallest group of particles that create a crystal system

Allotropes

• Two or more different molecular forms of the same element in the same physical state.

• They have different properties because their structures are different, even though they are all made up of the same element.

• Example: Diamond vs. Graphite (carbon)

Objective

19. Explain how three allotropes of carbon produce different functions.

• Diamond, graphite and fullerenes are all composed of carbon, but have different molecular structures. Because of this the physical properties are different.

• Diamond has a high density and is very hard.• Graphite has a low density and is soft.• Fullerenes are strong and rigid.

Non-Crystalline Solid

• Amorphous solid- lacks an ordered internal structure; the atoms are randomly arranged (Ex: rubber, plastic and asphalt).

• Glass-The transparent fusion product of inorganic substances that have cooled to a rigid state without crystallizing