Angstrom

Definition and Measurement of Angstrom

• The angstrom is a metric unit of length equal to 10^-10 meters.
• It is one ten-billionth (US) of a meter, a hundred-millionth of a centimeter, 0.1 nanometer, or 100 picometers.
• The unit is named after Swedish physicist Anders Jonas Ångström (1814–1874).
• The symbol for angstrom is Å, a letter of the Swedish alphabet.
• It is often used in the natural sciences and technology to express sizes of atoms, molecules, microscopic biological structures, chemical bond lengths, crystal arrangements, and wavelengths of electromagnetic radiation.

Historical Background and Standardization

• In 1868, Anders Jonas Ångström created a chart of the spectrum of sunlight, expressing wavelengths in multiples of one ten-millionth of a millimeter.
• The unit became widely used in solar physics, astronomical spectroscopy, and atomic spectroscopy.
• Initially, the definition of the angstrom based on the meter was not accurate enough for spectroscopy work.
• In 1907, the International Union for Cooperation in Solar Research defined the international angstrom as precisely 1/6438.4696 of the wavelength of a specific spectral line.
• In 1960, the angstrom was redefined as exactly 0.1 nanometers when the meter itself was redefined in spectroscopic terms.
• The International Bureau of Weights and Measures (BIPM) has provided standardized values for Angstrom.
• The current definition is based on the speed of light in a vacuum.
• The unit is now considered a non-SI metric unit.

Relationship to SI Units

• The angstrom is not a formal part of the International System of Units (SI).
• The closest SI unit to the angstrom is the nanometer (10^-9 meters).
• The International Committee for Weights and Measures officially discourages the use of the angstrom.
• It is not mentioned in the 9th edition of the official SI document.
• The angstrom is also not included in the European Union's catalogue of units of measure.

Angstrom Star

• After the redefinition of the meter, the angstrom was formally redefined as 0.1 nanometers.
• There was briefly a need for a separate unit of comparable size defined directly in terms of spectroscopy.
• The Angstrom Star (Å*) was defined as 0.202901 times the wavelength of a specific spectral line.
• The unit was accurate to within 5 parts per million but was later deemed obsolete due to higher precision measuring equipment.
• The angstrom star is no longer in use.

Symbol and Codification

• The formal symbol for the angstrom is U+212B ANGSTROM SIGN.
• Unicode recommends using U+00C5 LATIN CAPITAL LETTER A WITH RING ABOVE instead.
• Before digital typesetting, the angstrom was sometimes written as A.U.
• The abbreviation a.u. can also refer to the atomic unit of length or the astronomical unit.
• The symbol for angstrom is Å, a separate letter of the Swedish alphabet.

Angstrom Data Sources