femtometer

Femtometer, symbol fm, is a unit of length in the International System of Units (SI). It denotes 10^-15 meters. The prefix femto- indicates one quadrillionth of a unit. The femtometer is a convenient scale for describing subatomic sizes encountered in nuclear and particle physics.

Nuclear dimensions are typically measured in femtometres. For a nucleus with mass number A, its approximate radius is R ≈ 1.2 fm × A^(1/3). Consequently, typical nuclear radii lie in the range of about 1 to 7 fm. The proton and neutron radii themselves are on the order of 0.84–0.88 fm. Spacing between nucleons inside a nucleus is commonly around 1 fm.

In high-energy experiments, distances at the femtometer scale are probed via scattering processes and studies of nucleon structure. These measurements reveal details of nuclear forces and subnucleon phenomena studied in particle physics. Distances at much larger scales, such as atomic or molecular dimensions, are expressed in picometers or angstroms, illustrating the separation between nuclear and atomic physics.

Conversion and relation to other units: 1 fm equals 10^-15 meters. Since 1 micrometer (μm) equals 10^-6 meters, it follows that 1 μm equals 10^9 fm. The femtometer is thus vastly smaller than everyday macroscopic lengths but remains a standard unit for expressing nuclear-scale distances in physics.