Metals and Their Properties

Mobile Electrons in Metals

• Mobile electrons are responsible for efficient heat conduction in metals.

Metal Alloys

• Alloys are substances that contain a mixture of elements and exhibit metallic properties.
• Substitutional alloys occur when some host metal atoms are replaced by atoms of similar size. (e.g., Brass - Cu is replaced by Zn, Sterling silver - 93% Ag, 7% Cu).
• Interstitial alloys form when small atoms occupy the holes within the closely packed metal structure. (e.g., Steel - C atoms in Fe crystal).
• Heterogeneous alloys have components that are not evenly dispersed.

Network Solids

• Network solids are atomic solids with directional covalent bonds that form giant molecules.
• Properties:
  • Brittle
  • Inefficient conductors of heat and electricity
  • Hard
  • Colorless
  • Insulators
• Key elements:
  • Carbon - Examples include Diamond and Graphite.
  • Silicon

Diamond

• The hardest naturally occurring substance.
• Each carbon atom is surrounded by a tetrahedral arrangement of other carbon atoms.
• Localized electron model:
  • Stable structure via covalent bonds.
  • Formed by sp3 hybridized carbon atomic orbitals.
• Molecular orbital theory:
  • Large gaps between filled and empty levels.
  • Electron transfer is difficult.

Graphite

• Slippery, black, and conductive of heat and electricity.
• Structure is based on layers of carbon atoms arranged in fused six-membered rings.
• Localized electron model:
  • Trigonal planar arrangement.
  • 120-degree bond angles.
  • sp2 hybridization - three sp2 orbitals on each carbon atom form σ bonds with three other carbon atoms. One unhybridized 2p orbital remains perpendicular to the plane.
• Molecular orbital theory:
  • All orbitals combine to form π MOs which contribute to graphite's stability.
  • Delocalized electrons enable high electrical conductivity.

Semiconductors

• Gap exists between occupied MOs (valence band) and unoccupied ones (conduction band).
• Electrons must enter the conduction band for electron transfer.
• Group IVA elements have band gaps ranging from 0.08 to 3.05 eV (7 to 300 kJ/mol).
• Band gaps exceeding 3.5 eV result in an insulator.

Semiconductors (contd.)

• Group IVA elements are semiconductors due to their 4 valence electrons.
• Inorganic semiconductors (like GaAs) tend to have an average of 4 valence electrons.

Doping in Semiconductors

• Doping manipulates the conductivity of semiconductors by introducing elements with different numbers of valence electrons.
• n-type semiconductor: Conductivity is increased by doping with atoms that have more valence electrons than the host crystal. n-type have more electrons, allowing negative charge to travel in the conductance band.
• p-type semiconductor: Conductivity is increased by doping with atoms having fewer valence electrons than the host crystal. p-type have fewer electrons, allowing “holes” to travel in the valence band.

Molecular Solids

• Exhibit strong covalent bonding within molecules but weak forces between molecules.
• Forces between molecules depend on the nature of the molecules.
  • CO2, I2, P4 and S8: No dipole moment; they possess London dispersion forces.

Diffraction of X-rays

• Bragg's Law: nλ = 2d sin θ
  • n: Integer
  • λ: Wavelength of X-rays
  • d: Distance between atoms
  • θ: Angle of incidence and reflection
• Diffraction occurs due to:
  • Constructive interference when parallel beam waves are in phase.
  • Destructive interference when waves are out of phase.
• The distance traveled by waves depends on the distance between atoms.
• A diffractometer is used for X-ray analysis of crystals.

Example of X-ray Diffraction

• X-rays with a wavelength of 1.54 Å were used to analyze an aluminum crystal.
• A reflection was observed at θ = 19.3 degrees.
• Assuming n = 1, the distance between the atomic planes (d) can be calculated using Bragg's Law.

Classification of Solids

• Ionic solids: Ions occupy the lattice points defining their structure.
• Molecular solids: Discrete covalently bonded molecules exist at each lattice point.
• Atomic solids: Atoms occupy the lattice defining its structure.
  • Metallic solids: Delocalized, nondirectional covalent bonding.
  • Covalent network solids: Atoms are bonded to each other by strong, directional covalent bonds.
  • Group 8A (18) solids: Noble gases, attracted by London dispersion forces.

Structures of Metals

• Closest packing assumes metal atoms are hard, uniform spheres.
• Spheres are packed in layers, with each successive layer occupying dimples formed by the previous layer.