Introduction
One of the foremost scientists of the 20th century, Niels Henrik
David Bohr was the first to apply the quantum theory, which
restricts the energy of a system to certain discrete values,
to the problem of atomic and molecular structure. He was a guiding
spirit and major contributor to the development of quantum mechanics and atomic physics.
His work on atomic theory was recognized by the Nobel Prize
for Physics in 1922.
Early life.
Bohr was born in Copenhagen on Oct. 7, 1885. His father, Christian
Bohr, professor of physiology at the University of Copenhagen,
was known for his work on the physical and chemical aspects
of respiration. His mother, Ellen Adler Bohr, came from a wealthy
Jewish family prominent in Danish banking and parliamentary
circles. Bohr's scientific interests and abilities were evident
early, and they were encouraged and fostered in a warm, intellectual
family atmosphere. Niels's younger brother, Harald, became a
brilliant mathematician.
Bohr distinguished himself at the University of Copenhagen,
winning a gold medal from the Royal Danish Academy of Sciences
and Letters for his theoretical analysis of and precise experiments
on the vibrations of water jets as a way of determining surface
tension. In 1911 he received his doctorate for a thesis on the
electron theory of metals that stressed the inadequacies of
classical physics for treating the behaviour of matter at the
atomic level. He then went to England, intending to continue
this work with Sir J.J. Thomson at Cambridge. Thomson never
showed much interest in Bohr's ideas on electrons in metals,
however, although he had worked on this subject in earlier years.
Bohr moved to Manchester in March 1912 and joined Ernest Rutherford's
group studying the structure of the atom.
At Manchester Bohr worked on the theoretical implications of
the nuclear model of the atom recently proposed by Rutherford and known as the Rutherford atomic model.
Bohr was among the first to see the importance of the
atomic number, which indicates the position of an element in
the periodic table and is equal to the number of natural units
of electric charge on the nuclei of its atoms. He recognized
that the various physical and chemical properties of the elements
depend on the electrons moving around the nuclei of their atoms
and that only the atomic weight and possible radioactive behaviour
are determined by the small but massive nucleus itself. Rutherford's
nuclear
atom was both mechanically and electromagnetically unstable,
but Bohr imposed stability on it by introducing the new and
not yet clarified ideas of the quantum theory being developed
by Max Planck, Albert Einstein, and other physicists. Departing
radically from classical physics, Bohr postulated that any atom
could exist only in a discrete set of stable or stationary states,
each characterized by a definite value of its energy. This description of atomic structure is known as the Bohr atomic model.
The most impressive result of Bohr's essay at a
quantum theory of the atom was the way it accounted for the series of lines observed in the
spectrum of light emitted by atomic hydrogen. He was able to
determine the frequencies of these spectral lines to considerable
accuracy from his theory, expressing them in terms of the charge
and mass of the electron and Planck's constant (the quantum
of action, designated by the symbol
h). To do this, Bohr
also postulated that an atom would not emit radiation while
it was in one of its stable states but rather only when it made
a transition between states. The frequency of the radiation
so emitted would be equal to the difference in energy between
those states divided by Planck's constant. This meant that the
atom could neither absorb nor emit radiation continuously but
only in finite steps or quantum jumps. It also meant that the
various frequencies of the radiation emitted by an atom were
not equal to the frequencies with which the electrons moved
within the atom, a bold idea that some of Bohr's contemporaries
found particularly difficult to accept. The consequences of
Bohr's theory, however, were confirmed by new spectroscopic
measurements and other experiments.
Bohr returned to Copenhagen from Manchester during the summer
of 1912, married Margrethe Nørlund, and continued to
develop his new approach to the physics of the atom. The work
was completed in 1913 in Copenhagen but was first published
in England. In 1916, after serving as a lecturer in Copenhagen
and then in Manchester, Bohr was appointed to a professorship
in his native city. The university created for Bohr a new
Institute of Theoretical Physics, which opened its doors in
1921; he served as director for the rest of his life.
Through the early 1920s, Bohr concentrated his efforts on two
interrelated sets of problems. He tried to develop a consistent
quantum theory that would replace classical mechanics and electrodynamics
at the atomic level and be adequate for treating all aspects
of the atomic world. He also tried to explain the structure
and properties of the atoms of all the chemical elements, particularly
the regularities expressed in the periodic table and the complex
patterns observed in the spectra emitted by atoms. In this period
of uncertain foundations, tentative theories, and doubtful models,
Bohr's work was often guided by his
correspondence principle. According to this principle, every
transition process between stationary states as given by the
quantum postulate can be "coordinated" with a corresponding
harmonic component (of a single frequency) in the motion of
the electrons as described by classical mechanics. As Bohr put
it in 1923, "notwithstanding the fundamental departure from
the ideas of the classical theories of mechanics and electrodynamics
involved in these postulates, it has been possible to trace
a connection between the radiation emitted by the atom and the
motion of the particles which exhibits a far-reaching analogy
to that claimed by the classical ideas of the origin of radiation."
Indeed, in a suitable limit the frequencies calculated by the
two very different methods would agree exactly.
Bohr's institute in Copenhagen soon became an international
centre for work on atomic physics and the quantum theory. Even
during the early years of its existence, Bohr had a series of
coworkers from many lands, including H.A. Kramers from The Netherlands,
Georg Charles von Hevesy from Hungary, Oskar Klein from Sweden,
Werner Heisenberg from Germany, and John Slater from the United
States. Bohr himself began to travel more widely, lecturing
in many European countries and in Canada and the United States.
At this time, more than any of his contemporaries, Bohr stressed
the tentative and symbolic nature of the atomic models that
were being used, since he was convinced that even more radical
changes in physics were still to come.
In 1924 he was ready to consider the possibility that the conservation
laws for energy and momentum did not hold exactly on the atomic
level but were valid only as statistical averages. This extreme
measure for avoiding the apparently paradoxical particle-like
properties of light soon proved to be untenable and also unnecessary.
During the next few years, a genuine quantum mechanics was created,
the new synthesis that Bohr had been expecting. The new quantum
mechanics required more than just a mathematical structure of
calculating; it required a physical interpretation. That physical
interpretation came out of the intense discussions between Bohr
and the steady stream of visitors to his world capital of atomic
physics, discussions on how the new mathematical description
of nature was to be linked with the procedures and the results
of experimental physics.
Bohr expressed the characteristic feature of quantum physics
in his principle of complementarity, which "implies the impossibility
of any sharp separation between the behaviour of atomic objects
and the interaction with the measuring instruments which serve
to define the conditions under which the phenomena appear."
As a result, "evidence obtained under different experimental
conditions cannot be comprehended within a single picture, but
must be regarded as complementary in the sense that only the
totality of the phenomena exhausts the possible information
about the objects." This interpretation of the meaning of quantum
physics, which implied an altered view of the meaning of physical
explanation, gradually came to be accepted by the majority of
physicists. The most famous and most outspoken dissenter, however,
was
Einstein.
Einstein greatly admired Bohr's early work, referring to it
as "the highest form of musicality in the sphere of thought,"
but he never accepted Bohr's claim that quantum mechanics was
the "rational generalization of classical physics" demanded
for the understanding of atomic phenomena. Einstein and Bohr
discussed the fundamental questions of physics on a number of
occasions, sometimes brought together by a close mutual friend,
Paul Ehrenfest, professor of theoretical physics at the University
of Leiden, Neth., but they never came to basic agreement. In
his account of these discussions, however, Bohr emphasized how
important Einstein's challenging objections had been to the
evolution of his own ideas and what a deep and lasting impression
they had made on him.
During the 1930s Bohr continued to work on the epistemological
problems raised by the quantum theory and also contributed to
the new field of nuclear physics. His liquid-drop model of the atomic
nucleus, so called because he likened the nucleus to a liquid droplet, was a key step
in the understanding of many nuclear processes. In particular,
it played an essential part in 1939 in the understanding of
nuclear fission (the splitting of a heavy nucleus into two parts,
almost equal in mass, with the release of a tremendous amount
of energy). Similarly, his compound-nucleus model of the atom proved successful in explaining other types of nuclear reactions.
Bohr's institute continued to be a focal point for theoretical
physicists until the outbreak of World War II. The annual conferences
on nuclear physics as well as formal and informal visits of
varied duration brought virtually everyone concerned with quantum
physics to Copenhagen at one time or another. Many of Bohr's
collaborators in those years have written lovingly about the
extraordinary spirit of the institute, where young scientists
from many countries worked together and played together in a
lighthearted mood that concealed both their absolutely serious
concern with physics and the darkening world outside. "Even
Bohr," wrote H.B.G. Casimir, one of the liveliest of the group,
"who concentrated more intensely and had more staying power
than any of us, looked for relaxation in crossword puzzles,
in sports, and in facetious discussions."
Niels Bohr - Model of Atomic Structure:
Niels Bohr published his model of atomic structure in 1913. His theory was the first to
- that electrons traveled in orbits around the atom's nucleus
- that the chemical properties of the element was largely determined by the number of electrons in the outer orbits
- that an electron could drop from a higher-energy orbit to a lower one, emitting a photon (light quantum) of discrete energy
Niels Bohr model of atomic structure became the basis for all future quantum theories.
Werner Heisenberg and Niels Bohr:
In 1941, German scientist Werner Heisenberg made a secret and
dangerous trip to Denmark to visit his former mentor, physicist Niels
Bohr. The two friends had once worked together to split the atom until
World War II divided them. Werner Heisenberg worked on a German project
to develop atomic weapons, while Niels Bohr worked on the Manhattan
Project to create the first atomic bomb.
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