However, late 19th-century experiments with electric discharges had shown that atoms will only emit light (that is, electromagnetic radiation) at certain discrete frequencies. Also, as the electron spirals inward, the emission would rapidly increase in frequency due to the orbital period becoming shorter, resulting in electromagnetic radiation with a continuous spectrum. Rutherford's atom model is disastrous because it predicts that all atoms are unstable. Because the electron would lose energy, it would rapidly spiral inwards, collapsing into the nucleus on a timescale of around 16 picoseconds. the Larmor formula) predict that the electron will release electromagnetic radiation while orbiting a nucleus. This had electrons orbiting a solar nucleus, but involved a technical difficulty: the laws of classical mechanics (i.e. Given this experimental data, Rutherford naturally considered a planetary model of the atom, the Rutherford model of 1911. In the early 20th century, experiments by Ernest Rutherford established that atoms consisted of a diffuse cloud of negatively charged electrons surrounding a small, dense, positively charged nucleus. Origin Bohr model in 1921 after Sommerfeld expansion of 1913 model showing maximum electrons per shell with shells labeled in X-ray notation The quantum theory of the period between Planck's discovery of the quantum (1900) and the advent of a mature quantum mechanics (1925) is often referred to as the old quantum theory.

A related quantum model was proposed by Arthur Erich Haas in 1910 but was rejected until the 1911 Solvay Congress where it was thoroughly discussed. However, because of its simplicity, and its correct results for selected systems (see below for application), the Bohr model is still commonly taught to introduce students to quantum mechanics or energy level diagrams before moving on to the more accurate, but more complex, valence shell atom. As a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics and thus may be considered to be an obsolete scientific theory. The Bohr model is a relatively primitive model of the hydrogen atom, compared to the valence shell model. Not only did the Bohr model explain the reasons for the structure of the Rydberg formula, it also provided a justification for the fundamental physical constants that make up the formula's empirical results. While the Rydberg formula had been known experimentally, it did not gain a theoretical basis until the Bohr model was introduced. The model's key success lies in explaining the Rydberg formula for hydrogen's spectral emission lines. The improvement over the 1911 Rutherford model mainly concerned the new quantum mechanical interpretation introduced by Haas and Nicholson, but forsaking any attempt to explain radiation according to classical physics.

In the history of atomic physics, it followed, and ultimately replaced, several earlier models, including Joseph Larmor's Solar System model (1897), Jean Perrin's model (1901), the cubical model (1902), Hantaro Nagaoka's Saturnian model (1904), the plum pudding model (1904), Arthur Haas's quantum model (1910), the Rutherford model (1911), and John William Nicholson's nuclear quantum model (1912). It is analogous to the structure of the Solar System, but with attraction provided by electrostatic force rather than gravity, and with the electron energies quantized (assuming only discrete values). In atomic physics, the Bohr model or Rutherford–Bohr model of the atom, presented by Niels Bohr and Ernest Rutherford in 1913, consists of a small, dense nucleus surrounded by orbiting electrons. The 3 → 2 transition depicted here produces the first line of the Balmer series, and for hydrogen ( Z = 1) it results in a photon of wavelength 656 nm (red light). The orbits in which the electron may travel are shown as grey circles their radius increases as n 2, where n is the principal quantum number. The Bohr model of the hydrogen atom ( Z = 1) or a hydrogen-like ion ( Z > 1), where the negatively charged electron confined to an atomic shell encircles a small, positively charged atomic nucleus and where an electron jumps between orbits, is accompanied by an emitted or absorbed amount of electromagnetic energy ( hν). Not to be confused with Bohr equation or Bohr effect.

Bohr atomic model