Expanding the Kinetic Theory The nature of Solids Expanding the Kinetic Theory

Objectives When you complete this presentation, you will be able to … use kinetic theory to explain the properties of solids including melting point freezing point explain the significance of the crystal structure in solids define the unit cell of a particular crystalline structure define and give examples of solid allotropes

introduction Solids are more like liquids than gases. Solids are dense like liquids. Solids have a definite volume. However, solids also have a definite shape, which is not like either liquids or gases. We will continue to use the kinetic theory to explain the behavior of solids.

Assumptions The particles in a solid are very much more attracted to other particles than are particles in liquids or gases. This means that the individual solid particles … are so attracted to the other particles that it is very difficult for them to move very far.

Assumptions The particles in a solid are arranged in an orderly fashion in fixed locations. This means that the individual solid particles … are packed tight together to form the solid are packed so tight together that the solid is incompressible.

Assumptions The particles in a solid are in constant and rapid motion. This means that the individual solid particles … vibrate rapidly in place in the solid vibrate faster at higher temperatures and slower at lower temperatures

Melting Point When the temperature of a solid rises, the particles in the solid move faster and faster in place. At one temperature, the melting point, the solid particles start to move fast enough to become liquid particles. Likewise, as the temperature in a liquid falls to that same particular temperature, the freezing point, the liquid particles move slow enough to become solid particles.

Melting Point Solid Liquid For any particular compound, the melting point and the freezing point are the same value. We can have both solid and liquid at the temperature in equilibrium with individual particles moving from solid to liquid and back again. melting Solid Liquid freezing

Crystal Structures In a crystal, the particles are arranged in an orderly, repeating, 3-dimensional pattern. The shape of the crystal reflects the arrangement of the particles within the solid.

Crystal Structures There are seven crystal groups. Fluorite Rutile cubic hexagonal tetragonal rhombohedral orthorhombic monoclinic triclinic Fluorite Rutile Barite Rhodonite Corundum Cerussite Boron

Unit cell The shape of a crystal depends on the arrangement of the particles within it. The smallest group of particles within a crystal that retains the geometric shape of the crystal is the unit cell. A cubic structure has 3 different possible unit cells. Simple cubic Face centered cubic (fcc) Body centered cubic (bcc)

Unit cell Each of the crystal groups has a number of different kinds of unit cells. As we saw in the last slide, the cubic group can have three different kinds of unit cells. The tetragonal group can have two different kinds of cells. The monoclinic group can also have two different kinds of cells. The orthorhombic group can have four different kinds of cells. The rhombohedral, hexagonal, and triclinic groups have only one kind of cell each.

allotropes Allotropes are two or more different molecular forms of the same element in the same physical state For example, carbon. (a) graphite (b) diamond (c) buckminsterfullerene (d) carbon nanotube (e) graphene

allotropes Allotropes are two or more different molecular forms of the same element in the same physical state For example, phosphorus. (a) white phosphorus (b) black phosphorus (c) red phosphorus

Summary The particles in a solid are very much more attracted to other particles in the solid. The particles in a solid are arranged in an orderly fashion in fixed locations. The particles in a solid are in constant and rapid motion. For any particular compound, the melting point and the freezing point are the same value.

Summary In a crystal, the particles are arranged in an orderly, repeating, 3-dimensional pattern. There are 7 crystal groups. The smallest group of particles within a crystal that retains the geometric shape of the crystal is the unit cell. Allotropes are two or more different molecular forms of the same element in the same physical state