An Etymological Dictionary of Astronomy and Astrophysics
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The simplest model of an atom according to which electrons move around the central nucleus in circular, but well-defined, orbits. For more details see → Bohr model.

→ Bohr; → atom.

A type of cloud in the → interstellar medium with low molecular content that is fully exposed to the → interstellar radiation field, and therefore nearly all its → molecules are quickly destroyed by → photodissociation. Hydrogen is mainly in → neutral atomic form (→ neutral hydrogen), and atoms with → ionization potentials less than that of hydrogen (most notably → carbon) are almost fully → ionized, providing abundant electrons. The paucity of molecules implies that very little chemistry occurs in such clouds. Many → sightlines with low → extinction seem to pass exclusively through → diffuse atomic gas. Such sightlines typically have a → column density, N_H, less than about 5 × 10²⁰ cm^-2, and are sufficiently → optically thin to be observable by means of → visible and → ultraviolet → absorption line measurements. Diffuse atomic clouds typically have a fairly low → density (~ 10-100 cm^-3), and → temperatures of 30-100 K (Snow & McCall, 2006, ARA&A 44, 367).

→ diffuse; → atomic; → gas.

An atom in which one or more of its bound electrons are at → energy levels higher than their normal level.

Excited p.p. of → excite; → atom.

Between atoms; relating to the interaction of different atoms.

→ inter-; → atomic.

A weighted average of the time kept by about 200 caesium atomic clocks in over 50 national laboratories worldwide. It has been available since 1955, and became the international standard on which UTC is based on January 1972.

→ international; → atomic; → time.

In a → crystal, an imperfection caused by the presence of an extra atom that occupies a place outside the normal → lattice position. It may be the same type of atom as the others (self interstitial) or an → impurity atom.

→ interstitial; → atom.

An atom in which the number of → protons equals the number of → electrons and therefore has no net → electric charge.

→ neutral; → atom.

Lemaître's (1931) name for the early dense Universe, which later became known as the → Big Bang theory.

From L. primaevus "early in life," from primus "first," → prime, + aevum "an age," + → -al; → atom.

A system in which the nuclei of two colliding atoms approach each other closely for a brief lapse of time so that their electrons are arranged in atomic orbitals characteristic of a single atom with combined atomic number.

→ quasi-; → atom.

The ratio of the mass of an atom of the → chemical element to one-twelfth the mass of an atom of carbon-12. Because an element in nature is usually a mixture of isotopes, the relative atomic mass is also the weighted mean of the atomic masses of all the atoms in a particular sample of the element, weighted by isotopic abundance. In this sense, relative atomic mass was once known as → atomic weight.

→ relative; → atomic; → mass.

A simple model assuming that the positive charge of the atom is not distributed uniformly throughout the atom (unlike the → Thomson atom), but is concentrated in a minute center or nucleus, and the negative charge is distributed over a sphere of radius comparable with the atomic radius.

After the British physicist and chesmist Ernest Rutherford (1871-1937), who put forward this model in 1911; Nobel Prize in Chemistry in 1908; → atom.

An atom that has lost one electron and has become a positive ion.

→ single; → ionized; → atom.

Of, relating to, or being smaller than the atom; of or relating to the inside of the atom.

→ sub-; → atomic.

Any particle that is small compared to the size of the atom, e.g. an electron, proton, neutron, neutrino, quark, meson, all of which are either bosons or fermions.

→ subatomic; → particle.

The earliest theoretical description of the inner structure of atoms whereby an atom consists of a sphere of positive electricity of uniform density, throughout which is distributed an equal and opposite charge in the form of electrons. The diameter of the sphere was supposed to be of the order of 10^-8 cm, the magnitude found for the size of the atom. → Rutherford atom.

→ Thomson; → atom.

The hydrogen molecule composed of three atoms in which one of the atoms is ionized. The molecular ion H₃⁺ plays a key role in the chemistry of gaseous → interstellar medium. It reacts efficiently with almost any neutral atom or molecule to initiate a network of ion-neutral reactions. It is responsible for producing molecules such as OH, CO, and H₂O. The first detection of H₃⁺ in the interstellar medium, via two closely spaced absorption lines (doublet) near 3.668 μm, were reported in two dense → molecular cloud cores along the lines of sight to the embedded → young stellar objects W33A and GL2136 (Geballe & Oka 1996). Since then H₃⁺ has been detected in several molecular clouds. It is believed that H₃⁺ forms via → cosmic ray → ionization of H₂ producing H₂⁺, which quickly reacts to another H₂ molecule to form H₃⁺ ( H₂ + CR → H₂⁺ and H₂⁺ + H₂→ H₃⁺ + H + 1.7 eV).

Tri-, → three; → atomic; → hydrogen; → molecular; → ion.

An atom whose nuclei → decay by → radioactivity.

→ unstable; → atom.