Angstrom (Å) — unit of length for atomic, molecular, and optical scales

Overview

The ångström, written with the symbol Å, is a unit of length equal to 1×10−10 metres (one ten‑billionth of a metre). It is a convenient scale for describing structures at the atomic and molecular level and sits between the picometre and the nanometre in everyday use. The ångström is a historical unit rather than part of the International System of Units; the equivalent nanometre notation is often preferred in formal SI contexts (unit of length, 0.1 nm).

Value and notation

The numerical value of one ångström can be written in scientific form as 1×10⁻¹⁰ m or in exponential notation as 1E-10 m; discussions of notation and conventions appear in many sources (scientific notation). In writing, the name is sometimes anglicized to "angstrom" when the diacritic is not available.

Common uses and examples

Because the scale matches atomic dimensions, the ångström remains popular in fields that describe very small distances. Typical examples include:

• Atomic sizes: many atoms have radii on the order of 0.5–2 Å, so single atoms are often described by angstroms (atoms).
• Chemical bonds: covalent and ionic bond lengths commonly fall between about 1 and 2 Å (chemical bonds).
• Optical wavelengths: visible light wavelengths are often quoted in ångströms in older literature (approximately 4000–7000 Å) for spectra and spectroscopy (visible-light spectra).
• Materials and devices: some descriptions of crystal lattice spacings or very small features in microfabrication use angstroms for clarity (integrated circuits).

History and adoption

The name honors Anders Jonas Ångström, a 19th‑century Swedish physicist who made foundational measurements in spectroscopy and the study of solar radiation. The unit was adopted by some scientific communities because it matched the convenient scale of spectroscopic wavelengths and atomic spacings long before the nanometre became widespread.

Standards, distinctions and modern practice

Although widely understood and still in active use within certain research communities (notably crystallography, surface science and molecular spectroscopy), the ångström is not an SI base unit. Standards organizations encourage use of the nanometre for SI consistency, but many practitioners continue to use the ångström for its readability when discussing atomic-scale distances. Comparisons such as a human hair being on the order of 10⁶ Å provide intuitive scale references in popular explanations.

Notable facts

The angstrom bridges descriptive convenience and historical practice: it is small enough to express atomic detail without unnecessary zeros, yet familiar enough to appear in textbooks and instrument reports. When precision and SI conformity are required, convert ångströms to metres or nanometres, but expect to encounter Å routinely in older literature and in fields that study the very small.