The Prize:
The Nobel Prize in Physics in 1922 to Niels Henrik David Bohr “for his services in the investigation of the structure of atoms and of the radiation emanating from them.”
The Science:
Atomic theory dates back more than 7,000 years to the Ancient Greeks, who through thought experiments developed the idea that if you repeatedly split a substance in half, eventually you come to an indivisible molecule called the “atom.”
In the 13th century, Pseudo-Geber coined the theory of corpuscularianism, in which all physical matter was believed to be made of divisible particles (corpuscles). Isaac Newton and Robert Boyle combined Pseudo-Geber’s theory with alchemy to form a 16th century corpuscular theory of light. As the discipline of chemistry evolved, atomic theory and the idea of classical elements (earth, fire, air and water) was disproved by scientific thinkers. In 1803, John Dalton created the modern atomic theory, defining each element as consisting of distinct atoms that combine to form compounds. This theory was further validated by Robert Brown (who discovered “Brownian motion”), Albert Einstein, and physicist Jean Perrin. Scientists Dmitri Mendeleev and Lavoisier developed the first periodic table in 1869, arranging the distinct elements by their chemical properties and atomic number.
It was a long wait, however, until J.J. Thomson made the first discovery of a component of the atomic model—the electron—in 1897. Thomson showed that applying voltage between two electrodes in a vacuum generates a ray (known as a cathode ray) of negative-charged “corpuscles” (to use Thomson’s term). Thomson believed that electrons were the fundamental unit of matter, which he tried to prove despite knowing that atoms as a whole have no charge. To reconcile this contradiction, Thomson developed his “plum-pudding” model, wherein electron “plums” swam in a “pudding” of positive charge that neutralized the overall charge of the atom.
One of Thomson’s students, Ernest Rutherford, disproved the likeness of the atom to a fruit-filled dessert through his interpretation of the elegant “gold-foil experiment.” When positively charged alpha particles emitted from radium bromide are shot at a piece of gold foil only a few hundred atoms thick, instead of passing through the gold foil as predicted by the “plum-pudding” model, some alpha particles deflect with wide angles or reflect right back. To explain this, Rutherford proposed a new model in which a solar system of “planetary” electrons encircles a positively-charged central particle (the nucleus). This model reconciled how atoms can have a neutral charge but also deflect an alpha particle because of the large positively-charged nucleus.
What Rutherford’s model could not explain is how an accelerating negative charge (the electron) avoids emitting electromagnetic energy, a known property of accelerating charges. The planetary model also does not explain the observation that when atoms are excited, say by heat, they emit characteristic radiation spectra that can be observed by spectroscopy.
It was around this same time that quantum theory, developed by Max Planck and furthered by Albert Einstein, dawned on the world of physics. This model, for which Planck won the 1918 Nobel Prize, describes the behaviour of light and other electromagnetic waves. Light and other types of energy exist in only discrete bundles called quanta, with no half-measures. The size of the bundles is equal to Planck’s constant, h, a very small unit. Inspired by quantum theory, Neils Bohr developed his atomic model in 1913.
The Bohr model, also known as the Bohr-Rutherford model, starts with the planetary model, but incorporates quantum theory into the orbits of the electrons. Bohr postulated that like light energy, electrons have discrete amounts of energy. His model further suggested that electrons cannot travel around the positively-charged nucleus outside specific orbits that can be thought of as spherical “shells” at defined distances from the nucleus. These orbital shells are defined by a signature energy value, the quantum part of the theory.
In addition, the model states that an electron can only gain or lose energy by “jumping up” or “jumping down” into another energetic orbital shell. When an atom is excited, an electron can “jump up” into a new orbital sphere. Electrons that enter into a higher-energy orbital shell are unstable, and can not stay there for long. As they drop down to a lower orbit, they release energy in the form of radiation equivalent to the energy difference between the two orbital shells. These “jump downs” always release the same amount of radiation at frequencies that are characteristic for that atom.
Bohr’s model successfully explained many observations of the atom circulating at that time.
What Has Happened Since:
The Bohr-Rutherford model was the most advanced model of the atom for many years, but it was refined by others including Arnold Sommerfeld. It becomes less useful when applied to the properties and behaviours of large atoms. Despite this, the Bohr-Rutherford model is still taught to new science students before moving on to more complex orbital-theory models.
Current models for the movements of electrons include complex orbital patterns, wave-like behaviour, and the uncertainty principle—the concept that electrons aren’t normally at discrete foci and that their position can be best described in terms of probabilities.
We also have evidence that the nucleus is composed of more than just positively charged particles. Scientists now hypothesize that its ingredients list extends to quarks, bosons, and leptons.
Niels Bohr continued working in theoretical physics for the remainder of his life, contributing much to the field of atomic theory. During the Second World War he worked with the Americans on the Manhattan project, but was a strong advocate for a peaceful sharing of atomic knowledge and against a nuclear arms race. His son, Aage Bohr, went on to continue the family Nobel legacy, winning the Nobel Prize in Physics in 1975 with two others “for the discovery of the connection between collective motion and particle motion in atomic nuclei and the development of the theory of the structure of the atomic nucleus based on this connection.”
