#### What did Niels Bohr do? Information about Niels Bohr works, atomic theory, complementarity principle, liquid drop nuclear model and contributions to science.

**Niels Bohr Contributions To Science;** (1885-1962), Danish physicist and one of the greatest scientists and thinkers of all time. He founded the modern theory of atomic and molecular structure, made a decisive contribution to the theory of nuclear structure and reactions, and introduced into the logic of scientific thought the new fundamental concept of complementarity.

**Contributions to Science.** Bohr’s first great achievement, at the age of 28, had its starting point in the British physicist Ernest Rutherford’s discovery of the atomic nucleus. From this discovery emerged a model of the atom as a complex system consisting of an extremely small but dense nucleus carrying a positive electric charge, and a certain number of negatively charged electrons moving in definite orbits around the nucleus under the influence of the latter’s attraction.

Bohr saw at once that this model of atomic structure opened the way to a comprehensive theory of all physical and chemical properties of matter. To this end, however, a radical departure from the classical laws of mechanics and electromagnetism was called for, since these laws were unable to account for the stability of atomic systems of the type proposed by Rutherford. According to the classical law a Rutherford atom would be expected to decay in a very short time owing to radiation of energy by the moving electrons. Bohr realized that atomic stability was related to the feature of discontinuity which Planck had discovered in the structure of electromagnetic radiation. He postulated that the atom is capable of subsisting without radiating energy in a series of discrete stationary states, characterized by definite numbers of quanta of action. He further postulated that the atom radiates energy only when it makes a complete transition from one stationary state to another by emitting one quantum of energy in the form of electromagnetic radiation.

In this primitive form, Bohr’s theory was imperfect from the logical point of view, since the classical laws still had to be applied concurrently with the quantum postulates, which they contradicted. Useful guidance was derived from a general correspondence between the classical and quantum properties, but a logically consistent formulation of the whole theory was found only after many years of effort. In this period, Bohr’s criticism proved invaluable. Werner Heisenberg’s matrix mechanics, which brought about the solution of the dilemma in 1925, may be regarded as the successful conclusion of a line of investigation inspired by Bohr.

Enriched by essential contributions from an independent line of thought initiated by Louis de Broglie and perfected by Erwin Schrôdinger, the new quantum mechanics succeeded in incorporating the quantum postulates in a generalization of mechanics and electromagnetism. Nevertheless, its formulation still presented serious logical difficulties, inasmuch as it purported to be a synthesis of apparently conflicting classical and quantum features of the phenomena. At this juncture, Bohr’s intervention, in 1927, was again decisive in providing a complete elucidation of this logical problem and pointing out its universal significance.

**The Complementarity Principle.** It may be said that this work of Bohr’s inaugurated a new phase in the evolution of human thinking. Indeed, he was led to recognize that the traditional conception of deterministic causality was in need of generalization when applied to atomic processes.

The latter may be considered from two points of view—space-time localization and momentum-energy conservation—which, although contradictory and mutually exclusive, are nevertheless both indispensable to give a complete description of experience. Yet the domains of validity of each of these two modes of description are subject to a reciprocal limitation, which prevents any logical contradiction. This means, however, that the predictions that can be made on the outcome of a definite experiment concern, in general, probabilities of various possible results. To this novel type of statistical causality Bohr gave the name “complementarity”: the two mutually exclusive modes of description are said to be complementary to each other, and they condition each other in a statistical way.

The recognition of relations of complementarity between classical and quantal concepts, expressing their mutual limitations, makes it possible to give a complete and consistent account of atomic phenomena in terms of these concepts. On many occasions Bohr stressed the possibility of applying a similar method of analysis to the most varied domains of knowledge, in particular to biology, where apparent contradictions are found to resolve themselves into relations of complementarity.

**The Liquid-Drop Nuclear Model.** After about 1930, the research activity in Bohr’s institute in Copenhagen was more and more concentrated on the exploration of the atomic nucleus, a development made possible by the continual refinement of experimental techniques. In 1936, Bohr made an essential contribution to this new domain by proposing a general picture of the constitution of nuclei and of their transmutations and disintegrations.

This picture remains one of the foundations of the analysis of nuclear reactions. Once again, he succeeded in establishing order and harmony in a situation in which different empirical data pointed to seemingly contradictoiy features in the behavior of nuclear systems. He pointed out that, although the nuclear processes, just as the atomic ones, are governed by the quantum postulates, the smallness of the region in which the nuclear interactions take place, as well as the strength of these interactions, justify an approximate description of the transition processes much nearer to the classical limit than in the case of atoms.

According to this picture, a nuclear reaction occurs in two stages, the first of which is the capture of the particle with which the nucleus is bombarded, leading to the formation of a compound system of great energy and long lifetime (on the nuclear scale) comparable to a liquid droplet of very high temperature. In the second stage, this compound system gets rid of its excess energy by various processes, such as emission of particles of radiation, which may be compared to the evaporation of the hot droplet. One of the possible processes, for heavy nuclei, is the fission of the compound system into two fragments of comparable sizes. It is to Bohr that we owe the understanding of the mechanism of nuclear fission, described by him immediately after its discovery in 1939.