Hundred years of the Bohr atom model

Neil Bohr’s atomic model changed the world of Qauntum Physics. It laid the key foundation for the full quantum understanding of atomic and molecular spectroscopy and led him to the prestigious Nobel prize, writes C Sivaram

Just one hundred years ago this month, i.e. in July 1913, Danish physicist Niels Bohr published his landmark paper on the quantum model of the atom which laid the foundation for our understanding of atomic spectra apart from being a major step in the early development of quantum theory.

He held a lectureship at the Copenhagen University when the paper was published, but the origin and development of the relevant ideas go back to the time of his fruitful stay with Rutherford in Manchester. Prior to this, Bohr first came to England in September 1911 to work with J J Thomson (the discoverer of the electron) in Cambridge. When he arrived, he had very little knowledge of English which he tried to improve by reading through Charles Dickens’ Pickwick Papers with the help of a dictionary! In 1904, Thomson had proposed a “plum pudding” model of the atom in which, the atom is formed by electrons (plums) arranged with a spherical, positively charged matrix (pudding) so that the whole is electrically neutral.

Even before Bohr arrived at Cambridge, Thomson’s model was under a severe challenge as a result of Ernest Rutherford’s analysis for the famous gold foil experiment which he had directed at the University of Manchester. The experiments on scattering of alpha particles, seemed to show that most of an atom’s mass was concentrated in a nucleus, leaving a central positive charge. Rutherford’s main interest in this experiment had been the mechanism of scattering rather than the structure of the atom.

Nevertheless, Bohr did start his work with Thomson at Cambridge, but found himself primarily undertaking experimental work on Cathode rays. Moreover, Thomson was so busy with his own work that he was disinclined to even read Bohr’s doctoral thesis, let alone interact. Bohr thus decided to join Rutherford at Manchester to “discover more about radioactivity” and arrived there in March 1912, having hardly spent six months at Cambridge. Bohr’s time in England primarily switched his focus from classical physics to quantum theory.

After attending a course on radioactivity, Bohr was directed by Rutherford to carry out an experiment to study absorption of alpha particles by aluminium. Although Rutherford was more approachable than Thomson, Bohr was keen to focus more on theory and within a few weeks had put the aluminium aside for pencil and paper! He initially worked on the concept of isotopes believing that the defining factor for an element was not its atomic mass, but the atomic number, which is the positive charge of the nucleus. While studying the energy loss of alpha particles interacting with atomic electrons, Bohr considered the possibility that they were bound to the nucleus via some kind of elastic connection implying that they should vibrate with specific frequencies as suggested by Planck’s 1900 quantum theory of radiation.

In June 1912, he made the connection that the radiation of energy from these electrons could be limited by Plank’s constant thus bringing quantum theory into atomic structure for the first time. He prepared a draft for Rutherford called “On the constitution of atoms and molecules”, the same title as his final paper, where, for the first time, he dared to take on the implication of Rutherford’s tiny nucleus for the structure of the atom. In his model, Bohr had electrons in classical orbits around the positively charged nucleus, but Maxwell’s theory had shown that such electrons, with their centripetal acceleration, would give off electromagnetic radiation and collapse onto the nucleus in no time. Rather than give up at that point, Bohr took the mental leap of postulating that electrons orbiting the central nucleus achieved stability by having their angular momenta in quantized units of the Planck constant.

What Planck and Einstein had achieved for explaining the blackbody radiation and the photoelectric effect respectively by postulating that radiation is emitted or absorbed in discrete (quantized) units — the energy being integral multiples of the Planck constant times the frequency — Bohr now achieved for atomic spectroscopy by postulating that the orbital angular momenta for the electron orbits are quantized in multiples of n times the Planck constant. In 1913, Bohr also became aware of the work of Jakob Balmer that fitted a simple formula to all the spectral lines of hydrogen.

Bohr assumed that in these stationery orbits, where the angular momenta are quantized, electrons do not radiate classically. When the atom emitted or absorbed radiation of frequency f, the electron jumped from one orbit to another, the energy emitted or absorbed by each jump being equal to the frequency times Planck constant.
Bohr justifiably got the Physics Nobel prize in 1922 for the breakthrough work in explaining atomic spectra done in July 1913. Bohr, with his seminal atomic model, laid the key foundation for a full quantum understanding of atomic and molecular spectroscopy.

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