1 Solids

2 Structures of Solids Crystalline vs. Amorphous Crystalline solid: well-ordered, definite arrangements of molecules, atoms or ions. –Most solids are crystalline: iron, sand, salt, sugar Amorphous solid: random arrangements of molecules, atoms, or ions. –A few solids are amorphous: glass, rubber, cotton candy, butter –Amorphous solids usually solidify too fast for an orderly arrangement to form

3 Structures of Crystalline Solids Unit Cells Crystalline solid: well-ordered, definite arrangements of molecules, atoms or ions. Crystals have an ordered, repeated structure. The smallest repeating unit in a crystal is a unit cell. Unit cell is the smallest unit with all the symmetry of the entire crystal. Three-dimensional stacking of unit cells is the crystal lattice.

4 Structures of Solids Unit Cells

5 Structures of Solids Three Types of Unit Cells Primitive cubic –atoms at the corners of a simple cube –each atom shared by 8 unit cells; so 1 atom per unit cell Body-centered cubic (bcc), –atoms at the corners of a cube plus one in the center of the body of the cube –corner atoms shared by 8 unit cells, center atom completely enclosed in one unit cell; so 2 atoms per unit cell

6 Structures of Solids Three Types of Unit Cells Face-centered cubic (fcc), –atoms at the corners of a cube plus one atom in the center of each face of the cube –corner atoms shared by 8 unit cells, face atoms shared by 2 unit cells; so 4 atoms per unit cell

7 Structures of Solids Three Types of Unit Cells

8 Structures of Solids Crystal Structure of Sodium Chloride Face-centered cubic lattice. Two equivalent ways of defining unit cell: –Cl - (larger) ions at the corners of the cell, or –Na + (smaller) ions at the corners of the cell. The cation to anion ratio in a unit cell is the same for the crystal. In NaCl each unit cell contains same number of Na + and Cl - ions. Note the unit cell for CaCl 2 needs twice as many Cl - ions as Ca 2+ ions.

9 Structures of Solids Crystal Structure of Sodium Chloride

10 Structures of Solids X-Ray Diffraction When waves are passed through a narrow slit they are diffracted. When waves are passed through a diffraction grating (many narrow slits in parallel) they interact to form a diffraction pattern (areas of light and dark bands). Efficient diffraction occurs when the wavelength of light is close to the size of the slits. The spacing between layers in a crystal is Å, which is the wavelength range for X-rays.

11 Structures of Solids X-Ray Diffraction

12 Structures of Solids X-Ray Diffraction X-ray diffraction (X-ray crystallography): –X-rays are passed through the crystal and are detected on a photographic plate. –The photographic plate has one bright spot at the center (incident beam) as well as a diffraction pattern. –Each close packing arrangement produces a different diffraction pattern. –Knowing the diffraction pattern, we can calculate the positions of the atoms required to produce that pattern. –We calculate the crystal structure based on a knowledge of the diffraction pattern.

13 Bonding in Solids There are four types of solid: – Molecular (formed from molecules): usually soft with low melting points and poor conductivity. –Covalent network (formed from atoms): very hard with very high melting points and poor conductivity. –Ions (formed form ions): hard, brittle, high melting points and poor conductivity. –Metallic (formed from metal atoms): soft or hard, high melting points, good conductivity, malleable and ductile.

14 Bonding in Solids

15 Bonding in Molecular Solids Intermolecular forces: dipole-dipole, London dispersion and H-bonds. Weak intermolecular forces give rise to low melting points. Room temperature gases and liquids usually form molecular solids at low temperature. Efficient packing of molecules is important (since they are not regular spheres).

16 Bonding in Covalent Network Solids Covalent Bonding. Atoms held together in large networks. Examples: diamond, graphite, quartz (SiO 2 ), silicon carbide (SiC), and boron nitride (BN). In diamond: –each C atom has a coordination number of 4; –each C atom is tetrahedral; –there is a three-dimensional array of atoms. –Diamond is hard, and has a high melting point (3550° C).

17 Bonding in Covalent Network Solids

18 Bonding in Covalent Network Solids In graphite –each C atom is arranged in a planar hexagonal ring; –layers of interconnected rings are placed on top of each other; –the distance between C atoms is close to benzene (1.42 Å vs Å in benzene); –the distance between layers is large (3.41 Å); –electrons move in delocalized orbitals (good conductor).

19 Bonding in Ionic Solids Ions (spherical) held together by electrostatic forces of attraction: The higher the charge (Q) and smaller the distance (d) between ions, the stronger the ionic bond. There are some simple classifications for ionic lattice types:

20 Bonding in Ionic Solids

21 Bonding in Ionic Solids –NaCl Structure Each ion has a coordination number of 6. Face-centered cubic lattice. Cation to anion ratio is 1:1. Examples: LiF, KCl, AgCl and CaO. –CsCl Structure Cs + has a coordination number of 8. Different from the NaCl structure (Cs + is larger than Na + ). Cation to anion ratio is 1:1.

22 Bonding in Ionic Solids Zinc Blende Structure –Typical example ZnS. –S 2- ions adopt a fcc arrangement. –Zn 2+ ions have a coordination number of 4. –The S 2- ions are placed in a tetrahedron around the Zn 2+ ions. –Example: CuCl. Fluorite Structure –Typical example CaF 2. –Ca 2+ ions in a fcc arrangement. –There are twice as many F - per Ca 2+ ions in each unit cell. –Examples: BaCl 2, PbF 2.

23 Bonding in Metallic Solids Metallic solids have metal atoms in hcp, fcc or bcc arrangements (only Po is simple cubic packing). Coordination number for each atom is either 8 or 12. Problem: the bonding is too strong for London dispersion and there are not enough electrons for covalent bonds. Resolution: the metal nuclei float in a sea of electrons. Metals conduct because the electrons are delocalized and are mobile.

24 Bonding in Metallic Solids