solids classified into two general types: a.crystalline b.amorphorous

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.

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 CrystalsType Bonding Characteristic

sExamples

Metallic Metallic bonds Excellent conductor, high melting point

Silver

Copper

Ionic Electrostatic Ionic bonds

Brittle, poor conductors

NaCl

CuSO4

Molecular Intermolecular Forces

Soft, low melting point, poor conductors

H2O

Cholesterol

Network Covalent 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.

7-17-1 SolidsSolids

CUBIC CRYSTAL

BODY-CENTERED CUBIC CRYSTAL

FACE-CENTERED CUBIC CRYSTAL

7-17-1 SolidsSolids

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.

l4

2r

Name # atoms Length of side (l)

Volume

Simple Cubic 8 corners X 1/8 =1 2r = l l3

Body Centered Cubic

8 corners X 1/8 =11 body atom =1 2

l3

Face Centered Cubic

8 corners X 1/8 =16 faces X ½ =3 4

l3

l43

r

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 0.8560 g/cm3 and it has a BCC crystal structure