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SolidsSolids

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Structures of SolidsStructures of SolidsCrystalline vs. AmorphousCrystalline 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

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Structures of Crystalline SolidsStructures of Crystalline SolidsUnit CellsUnit 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.

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Structures of SolidsStructures of SolidsUnit CellsUnit Cells

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Structures of SolidsStructures of SolidsThree Types of Unit CellsThree 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

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Structures of SolidsStructures of Solids

Three Types of Unit CellsThree 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

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Structures of SolidsStructures of SolidsThree Types of Unit CellsThree Types of Unit Cells

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Structures of SolidsStructures of SolidsCrystal Structure of Sodium ChlorideCrystal 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 CaCl2 needs twice as many Cl- ions as Ca2+ ions.

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Structures of SolidsStructures of SolidsCrystal Structure of Sodium ChlorideCrystal Structure of Sodium Chloride

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Structures of SolidsStructures of SolidsX-Ray DiffractionX-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 2 - 20 Å, which is the wavelength range for X-rays.

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Structures of SolidsStructures of SolidsX-Ray DiffractionX-Ray Diffraction

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Structures of SolidsStructures of SolidsX-Ray DiffractionX-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.

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Bonding in SolidsBonding 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.

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Bonding in SolidsBonding in Solids

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Bonding in Molecular SolidsBonding 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).

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Bonding in Covalent Network SolidsBonding in Covalent Network Solids• Covalent Bonding.• Atoms held together in large networks.• Examples: diamond, graphite, quartz (SiO2), 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).

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Bonding in Covalent Network SolidsBonding in Covalent Network Solids

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Bonding in Covalent Network SolidsBonding 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. 1.395 Å in benzene);– the distance between layers is large (3.41 Å);– electrons move in delocalized orbitals (good conductor).

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Bonding in Ionic SolidsBonding 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:

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Bonding in Ionic SolidsBonding in Ionic Solids

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Bonding in Ionic SolidsBonding 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.

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Bonding in Ionic SolidsBonding in Ionic Solids• Zinc Blende Structure– Typical example ZnS.– S2- ions adopt a fcc arrangement.– Zn2+ ions have a coordination number of 4.– The S2- ions are placed in a tetrahedron around the Zn2+

ions.– Example: CuCl.

• Fluorite Structure– Typical example CaF2.– Ca2+ ions in a fcc arrangement.– There are twice as many F- per Ca2+ ions in each unit cell.– Examples: BaCl2, PbF2.

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Bonding in Metallic SolidsBonding 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.

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Bonding in Metallic SolidsBonding in Metallic Solids