Basic Concepts
Key concepts in atomic-scale measurements:
- Angstrom (Å): Named after physicist Anders Jonas Ångström, equals 10-10 meters
- Nanometer (nm): Equals 10-9 meters or 10 angstroms
- Atomic dimensions: Most atoms have diameters of 1-5 Å
- Bond lengths: Typical chemical bonds range from 0.74 Å (H-H) to 2.8 Å (C-I)
- Wavelength of light: Visible light ranges from 4000-7000 Å (400-700 nm)
- X-ray wavelengths: Typically 0.1-100 Å, ideal for probing atomic structures
Applications
How these measurements are used in science:
- X-ray Crystallography: Determines atomic positions in crystals using angstrom-scale measurements
- Electron Microscopy: Images structures at nanometer and sub-nanometer resolution
- Material Science: Characterizes nanomaterials and thin films
- Molecular Biology: Measures protein structures and DNA dimensions
- Semiconductor Industry: Designs and fabricates nanoscale electronic components
- Quantum Chemistry: Calculates molecular geometries and electron distributions
Crystal Structures
Understanding atomic arrangements in solids:
- Unit cell: The smallest repeating structural unit in a crystal
- Lattice parameters: Dimensions (a, b, c) and angles (α, β, γ) defining the unit cell
- Miller indices: System for identifying crystal planes and directions
- Bravais lattices: 14 possible three-dimensional lattice arrangements
- Nearest neighbor distance: Shortest distance between adjacent atoms
- Packing efficiency: Percentage of space occupied by atoms in a crystal
Common Values
Reference data for atomic-scale measurements:
- Hydrogen atom radius: ~0.53 Å (Bohr radius)
- Carbon-carbon bond: ~1.42 Å (in graphite)
- DNA double helix width: ~20 Å (2 nm)
- Silicon crystal lattice: 5.43 Å
- Gold atom diameter: ~2.88 Å
- Water molecule size: ~2.75 Å
Measurement Techniques
Methods for measuring at the atomic scale:
- X-ray Diffraction (XRD): Determines crystal structures with angstrom precision
- Transmission Electron Microscopy (TEM): Achieves sub-angstrom resolution
- Scanning Tunneling Microscopy (STM): Images individual atoms on surfaces
- Atomic Force Microscopy (AFM): Maps surface topography at nanometer scale
- Small-Angle X-ray Scattering (SAXS): Analyzes nanoscale structures
- Neutron Diffraction: Locates light atoms like hydrogen in crystal structures
Historical Context
Evolution of atomic-scale measurements:
- 1814: First estimation of atomic size by Thomas Young
- 1878: Ångström unit proposed by Anders Jonas Ångström
- 1912: First X-ray diffraction experiment by Max von Laue
- 1913: Bragg's Law formulated, enabling precise crystal structure determination
- 1960: International System of Units (SI) adopts the nanometer
- 1981: Invention of STM enables direct visualization of individual atoms