Solids Classified into two general types: a.Crystalline b.amorphorous

Amorphous Solids An "amorphous solid" is a solid in which there is no long-range order of the positions of the atoms.solidlong-range orderatoms Most classes of solid materials can be found or prepared in an amorphous form. Prepared by fast cooling. Molecules are frozen in place when the phase changes. Glass is the most famous example.

Crystalline Solids When cooled slowly, atomic and molecular builidng blocks assembled in well ordered, low energy structures called crystals. Examples

Types of Crystals TypeBondingCharacteristicsExamples MetallicMetallic bondsExcellent conductor, high melting point Silver Copper IonicElectrostatic Ionic bonds Brittle, poor conductors NaCl CuSO 4 MolecularIntermolecular Forces Soft, low melting point, poor conductors H 2 O Cholesterol NetworkCovalent network Hard, brittle, high melting point Diamond

Crystal Vocabulary Lattice- a three dimensional system of points designating the position of the components (ions, atoms or molecules) that make up the substance. Unit Cell- The smallest repeating unit of the lattice.

IONIC CRYSTALS In ionic crystals, ions pack themselves so as to maximize the attractions and minimize repulsions between the ions. 7-1 Solids

CUBIC CRYSTAL

BODY-CENTERED CUBIC CRYSTAL

FACE-CENTERED CUBIC CRYSTAL 7-1 Solids

8–10 Packing of Spheres and the Structures of Metals Arrays of atoms act as if they are spheres. Two or more layers produce 3-D structure. Two cubic arrays one directly on top of the other produces simple cubic (primitive) unit cell. Offset layers produces a-b-a-b arrangement since it takes two layers to define arrangement of atoms. Called cubic closest packed. Makes a body centered cubic unit cell. –.

8–11 Packing of Spheres and the Structures of Metals Hexagonal closest packed requires three layers to make a repeating pattern (abc, abc, …). It forms a face centered cubic unit cell.

Determining number of spheres in a unit cell. Corner = 1/8 sphere, each unit cell contains 8 corners Face = 1/2 sphere, each face centered unit cell 6 corners Body = 1 sphere, each body centered unit cell contains 1 sphere

Determine the number of spheres in a face centered cubic unit cell. There are 8 corners. There are 6 faces. There are no body spheres. (8 X 1/8) + (6 X ½) = 4 spheres

8–14 Cubic Unit Cells in Crystalline Solids Primitive-cubic shared atoms are located only at each of the corners. 1 atom per unit cell. Body-centered cubic 1 atom in center and the corner atoms give a net of 2 atoms per unit cell. Face-centered cubic corner atoms plus half-atoms in each face give 4 atoms per unit cell.

8–15 Calculations involving the Unit Cell The density of a metal can be calculated if we know the length of the side of a unit cell. Name# atomsLength of side (l) Volume Simple Cubic8 corners X 1/8 =1 2r = l l3l3 Body Centered Cubic 8 corners X 1/8 =1 1 body atom =1 2 l3l3 Face Centered Cubic 8 corners X 1/8 =1 6 faces X ½ =3 4 l3l3

8–16 Polonium crystallizes according to the simple cubic structure. Determine its density if the atomic radius is 167 pm.

8–17 Calculate the radius of potassium if its density is g/cm 3 and it has a BCC crystal structure