The nature of Solids

1. The nature of Solids Expanding the Kinetic Theory

2. 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

3. introduction • Solids are more like liquids than gases. • Solids are denselike 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.

4. 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.

5. 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.

6. 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

7. 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.

8. Melting Point • 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

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

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

11. 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)

12. 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. Thetetragonal group can have twodifferent kinds of cells. The monoclinicgroup can also have twodifferent kinds of cells. The orthorhombicgroup can have fourdifferent kinds of cells. The rhombohedral, hexagonal, and triclinicgroups have only onekind of cell each.

13. allotropes • Allotropesare two or more differentmolecular forms of the sameelement in the same physical state • For example, carbon. (a) graphite (b) diamond (c) buckminsterfullerene (d) carbon nanotube (e) graphene

14. allotropes • Allotropesare two or more differentmolecular forms of the sameelement in the same physical state • For example, phosphorus. (a) white phosphorus (b) black phosphorus (c) red phosphorus

15. 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.

16. 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