An Etymological Dictionary of Astronomy and Astrophysics
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Niels Bohr (1885-1962), Danish physicist who made several important contributions to modern physics. He won the 1922 Nobel prize for physics in recognition of his work on the structure of atoms.

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 fundamental constant, first calculated by Bohr, for the intrinsic → spin magnetic moment of the electron. It is given by: μ_B = eħ/2m_e = 9.27 x 10^-24 joule/tesla = 5.79 x 10^-5 eV/tesla, representing the minimum amount of magnetism which can be caused by the revolution of an electron around an atomic nucleus. It serves as a unit for measuring the magnetic moments of atomic particles.

→ Bohr; magneton, from → magnet + → -on.

A model suggested in 1913 to explain the stability of atoms which classical electrodynamics was unable to account for. According to the classical view of the atom, the energy of an electron moving around a nucleus must continually diminish until the electron falls onto the nucleus. The Bohr model solves this paradox with the aid of three postulates (→ Bohr's first postulate, → Bohr's second postulate, → Bohr's third postulate). On the whole, an atom has stable orbits such that an electron moving in them does not radiate electromagnetic waves. An electron radiates only when making a transition from an orbit of higher energy to one with lower energy. The frequency of this radiation is related to the difference between the energies of the electron in these two orbits, as expressed by the equation hν = ε₁ - ε₂, where h is → Planck's constant and ν the radiation frequency. The electron needs to gain energy to jump to a higher orbit. It gets that extra energy by absorbing a quantum of light (→ photon), which excites the jump. The electron does not remain on the higher orbit and returns to its lower energy orbit releasing the extra energy as radiation. Bohr's model answered many scientific questions in its time though the model itself is oversimplified and, in the strictest sense, incorrect. Electrons do not orbit the nucleus like a planet orbiting the Sun; rather, they behave as → standing waves. Same as → Bohr atom.

→ Bohr; → model.

The radius of the orbit of the hydrogen electron in its ground state (0.529 Å).

→ Bohr; → radius.

One of the postulates used in the → Bohr model, whereby there are certain steady states of the atom in which electrons can only travel in stable orbits. In spite of their acceleration, the electrons do not radiate electromagnetic waves when they move along stationary orbits.

→ Bohr; → first; → postulate.

One of the three postulates advanced in the → Bohr model which led to the correct prediction of the observed line spectrum of hydrogen atom. See also → Bohr's first postulate, → Bohr's second postulate, → Bohr's third postulate,

→ Bohr; → postulate.

One of the postulates used in the → Bohr model, whereby when an atom is in the steady state an electron travelling in a circular orbit should have → quantized values of the → angular momentum which comply with the condition p = n(h/2π), where p is the angular momentum of the electron, h is → Planck's constant, and n is a positive integer called → quantum number.

→ Bohr; → second; → postulate.

One of the postulates used in the → Bohr model, whereby the atom emits (absorbs) a quantum of electromagnetic energy (→ photon) when the electron passes from an orbit with a greater (lesser) n value to one with a lesser (greater) value. The energy of the quantum is equal to the difference between the energies of the electron on its orbits before and after the transition or "jump": hν = ε₁ - ε₂, where h is the → Planck's constant and ν the frequency of the transition.

→ Bohr; → third; → postulate.