In the field of atomic and molecular physics, an "atomic unit of length" is a unit of measurement that is used to express distances at the atomic and molecular scale in a dimensionless way. It is part of a system of atomic units (a.u.) that simplifies calculations involving fundamental physical constants and properties of atoms and molecules.
The atomic unit of length (a.u. of length) is defined in terms of the Bohr radius (a₀), which is a fundamental constant in atomic physics. The Bohr radius is approximately 0.52917721067 angstroms (Å) or 5.2917721067 x 10^-11 meters (m).
In atomic units, the Bohr radius is set to exactly 1 a.u. of length. Therefore, when using atomic units, distances are expressed relative to the Bohr radius, and the value of 1 a.u. of length corresponds to the typical size scale of atomic and molecular structures.
The use of atomic units simplifies many quantum mechanical calculations and allows physicists and chemists to work with dimensionless quantities, making it easier to compare and analyze atomic and molecular properties.
A "link" is a unit of length that is equivalent to 1/100th of a chain. In the United States customary system of measurement, one link is equal to 7.92 inches or 0.201168 meters. It is primarily used in surveying and land measurement.
Links are often used in conjunction with other surveying units like chains and rods to measure distances on land, especially for large plots or parcels of land. The term "link" is rarely used in everyday conversation or modern applications outside of surveying and related fields. Instead, more common units like feet, yards, or meters are used for measuring length in most contexts.