Niels Bohr was one of the most influential scientists of the 20th Century and a major force in the field of quantum physics. He won the 1922 Nobel Prize in physics for his study of the structure and radiation of atoms. [See the animation of "The Bohr Atom", below, right.] Bohr recognized that Ernest Rutherford's model of the atom, in which electrons emitted radiation continuously, was unstable according to the laws of classical physics. Bohr postulated that radiation is emitted from atoms not as a result of the periodic motion of the electron in its orbit, but only when an electron "jumps" from one orbit to another losing energy that is emitted as radiation. Bohr's theory of the compound nucleus, in which the repulsion between positively charged protons is countered by huge amounts of energy in order to hold the nucleus together, helped lead to the hypothesis that splitting an atom would produce enough energy to fabricate a powerful weapon.

Bohr's father was a well-known Danish physiologist, his mother came from a wealthy family of Jewish bankers. Bohr earned his Ph.D. at the University of Copenhagen in 1911, then worked in Cambridge, England with J.J. Thomson, discoverer of the electron, and in Manchester with Rutherford, who proposed the first nuclear model of the atom.

In 1909 when his brother Harald left to pursue his own academic endeavors, Bohr hired Margrethe Norlund to type his numerous papers. One year later, the two became engaged and married in 1912. The marriage would produce six sons.

In 1921 Bohr was named director of the new Institute for Theoretical Physics in Copenhagen, which soon became a requisite destination for atomic physicists from around the world. During a lecture series in Gottingen, Germany, Bohr befriended the young Werner Heisenberg, [see video above] initiating their famous collaboration. In 1933, after the Nazis authorized German universities to fire staff on the grounds of politics and race, Bohr enabled young refugee physicists to come to his institute.

After Germany occupied Denmark, Bohr fled with his family to Sweden, and then to the United States where he helped develop the atomic bomb. As early as 1944, however, his concern about harnessing the power of nuclear energy led him to advocate control of nuclear weapons and world peace through the open sharing of knowledge among nations. He expressed these ideas to Winston Churchill and President Franklin Roosevelt, both of whom rejected his recommendations. After the war, he helped establish CERN, the European nuclear physics laboratory, and organized the first Atoms for Peace Conference in Geneva in 1955. He was the first recipient of the Atoms for Peace Award.

In 1956, Robert Jungk published a book, Brighter than a Thousand Suns, about the German atomic bomb effort during the war. The book included an excerpted letter from Werner Heisenberg detailing his recollection of his meeting with Bohr in Copenhagen in 1941. Heisenberg's recasting of the events so angered Bohr that he drafted a letter to Heisenberg renouncing his version of the meeting. However, the letter was never sent. But in February of 2002, in order to clarify Bohr's position about the meeting, the Bohr family posted on the internet the draft letter and other documents pertaining to the 1941 meeting.

Niels Bohr died in 1962 and is buried in Copenhagen.

Werner Heisenberg won the Nobel Prize in physics in 1932 for establishing the field of quantum mechanics. Heisenberg suggested that any theory of the atom must be based on observable phenomenon, such as the spectral lines emitted by atoms, and not pictorial constructs such as Bohr's nuclear model of the atom. For Heisenberg, this observable data could be culled to formulate a set of possible values for a hypothetical particle. These values could then be used to calculate, by mean of mathematical formulas, the probabilities of particular energy states and transitions among those states.

Quantum mechanics had a profound influence on the development of atomic and nuclear physics by providing a model for calculating such formulations as critical mass. Using mathematical laws of probability, nuclear physicists were able to determine how much fissionable material would be necessary to ensure the likelihood that enough neurons would collide with the material to cause fission, thus leading to the development of nuclear reactors and atomic bombs.

Heisenberg is best known for his uncertainty principle of 1927. The principle posits limits to the accuracy of knowledge about atomic behavior, since the means by which the researcher measures such phenomena -- short wave radiation bounced off a particle, for example -- alters the behavior itself.

The son of a university professor, Heisenberg studied theoretical physics under Arnold Sommerfeld at the University of Munich. During a 1922 lecture given by Niels Bohr at Gottingen, Heisenberg publicly questioned the mathematics of the Nobel Prize winner, earning Bohr's attention. Bohr invited Heisenberg for a hike, initiating their famous collaboration. Heisenberg received his doctorate in 1923, then went to work with Niels Bohr in Copenhagen. In 1927, Heisenberg returned to Germany to teach physics at the University of Leipzig.

Early in World War II, the deeply patriotic Heisenberg had conducted for the Germans chain reaction experiments with heavy water that led him to believe in the feasibility of a nuclear weapon. Heisenberg visited his mentor and former teacher, Niels Bohr, in Copenhagen in 1941, perhaps to ask his advice on the right course of action for the development of atomic energy. It is not certain what was said during this meeting, however Bohr came away with the impression that the Nazis were actively developing a nuclear bomb. Within a few days before Germany's surrender, Heisenberg was captured by Allied forces and incarcerated with other German atomic scientist at Farm Hall, a country estate near Cambridge, England. It was later determined that the Germans were never close to developing an atomic bomb. [For more on this topic, refer to the Journal of Chemical Education Webite.]

After World War II, Heisenberg directed the Max Planck Institute at Berlin, and then moved to the Max Planck Institute for Physics at Gottingen where he remained for the rest of his career.

In 1955, while working on a book about the development of atomic physics in Germany during the war, Robert Jungk queried Werner Heisenberg about his Copenhagen meeting with Niels Bohr. Heisenberg complied by sending a letter back recollecting what happened. An excerpted version of his letter was subsequently published in Jungk's book "Brighter Than a Thousand Suns."

Heisenberg's account of their meeting so upset Bohr that he dictated a letter to Heisenberg detailing his own, and very different, recollection of what had transpired. In the end, Bohr never sent his letter. In February of 2002, the Bohr family published Bohr's draft letter to Heisenberg on the Internet. The issue of what Heisenberg and Bohr said to one another in 1941 Copenhagen is still open for debate.

Werner Heisenberg died in Munich in 1976.

While Niels Bohr and Werner Heisenberg were meeting in Copenhagen in 1941, the American nuclear bomb program was still in its genesis. Colonel Leslie R. Groves, an Army Corps engineer who had just finished building the Pentagon, took charge of the program -- now called the Manhattan Project -- in September of 1942. Groves quickly hired Dr. J. Robert Oppenheimer, a brilliant physicist, to be the project's scientific director.

The site they chose was Los Alamos, New Mexico.

Construction on cheap barracks-like buildings not meant to outlast the war began almost immediately. Within three years, the Los Alamos Laboratory had designed and was very close to completing two prototype atomic bombs.

Meanwhile, the war in Europe had ground to an end. V-E Day was declared on May 8, 1945, with Germany in ruins and 39million people dead. With the war against Japan still raging in the Pacific, however, the pace at Los Alamos did not let up.

By the summer of 1945 a bomb was ready for testing. A flat and desolate area was chosen 200 miles south of Los Alamos near the Alamogordo Bombing Range. Oppenheimer named the test site "Trinity."

On July 16, 1945, at about 5:30 in the morning, soon after a thunderstorm had swept clean the area, the first nuclear bomb was exploded.

Why the Bohr family in February of 2002 decided to publish the documents.

"The family of Niels Bohr has decided to release all documents deposited at the Niels Bohr Archive, either written or dictated by Niels Bohr, pertaining specifically to the meeting between Bohr and Heisenberg in September 1941. There are in all eleven documents..."

Document Description
Introductory remarks as to the nature of the documents, including their possible dates and circumstances surrounding their creations.

"The material, covering the period from 1957 to 1962, is presented in high-resolution black-and-white facsimile reproductions of the original documents, original-language transcriptions and English translations..."

"...it was Bohr's definite impression that Heisenberg simply informed him - in general terms - of the existence of a German atomic bomb project and his own involvement in it...."

In 1897, J.J. Thomson discovered the electron, the first subatomic particle. He also was the first to attempt to incorporate the electron into a structure for the atom. The internal structure of the atom had been a source of speculation for thousands of years. The Greeks taught that the atom was solid, as did Dalton. Although Dalton did allow for the fact that there might be a sub-atomic structure of which he was unaware.

Thomson faced two major problems: (1) how to account for the mass of the atom when the electron was only about 1/1000 the mass of the hydrogen atom (the more modern figure is 1/1836) and (2) how to create a neutral atom when the only particle available was negatively charged.

His solution was to rule the scientific world for about a decade and Thomson himself would make a major contribution to undermining his own model.

Thomson had been in the business of proposing atomic models since at least 1881, which is when he proposed his "vortex" model of the atom. We will not go into details about it.

The first seed of the model we are discussing appear in his famous 1897 announcement of the discovery of the electron. He wrote:

"The explanation which seems to me to account in the most simple and straightforward manner for the facts is founded on a view of the constitution of the chemical elements which has been favourably entertained by many chemists: this view is that the atoms of the different chemical elements are different aggregations of atoms of the same kind. In the form in which this hypothesis was enunciated by Prout, the atoms of the different elements were hydrogen atoms; in this precise form the hypothesis is not tenable, but if we substitute for hydrogen some unknown primordial substance X, there is nothing known which is inconsistent with this hypothesis, which is one that has been recently supported by Sir Norman Lockyer for reasons derived from the study of the stellar spectra.

If, in the very intense electric field in the neighbourhood of the cathode, the molecules of the gas are dissociated and are split up, not into the ordinary chemical atoms, but into these primordial atoms, which we shall for brevity call corpuscles; and if these corpuscles are charged with electricity and projected from the cathode by the electric field, they would behave exactly like the cathode rays. "

And a few paragraphs later:

"If we regard the chemical atom as an aggregation of a number of primordial atoms, . . . ."

However, he does not go into the presence of a positive force, although he must have been aware of its necessisity.

Here is what he then said in 1899:

"I regard the atom as containing a large number of smaller bodies which I will call corpuscles, these corpuscles are equal to each other.... In the normal atom, this assemblage of corpuscles forms a system which is electrically neutral. Though the individual corpuscles behave like negative ions, yet when they are assembled in a neutral atom the negative effect is balanced by something which causes the space through which the corpuscles are spread to act as if it had a charge of positive electricity equal in amount to the sum of the negative charges of the corpuscles.... The detached corpuscles behave like negative ions, each carrying a constant negative charge which we shall call for brevity the unit charge; while the part of the atom left behind behaves like a positive ion with the unit positive charge and a mass large compared with that of the negative ion."

This last portion is interesting in that it proposes the correct mechanism for ionization; a negative electron is removed leaving behind a positive atom.

His next statement on the structure of the atom comes in a 1904 article. The first half of the article is filled with detailed calculations about the stability of corpuscles moving about in a positive environment. In fact, Thomson is only able to make calculations where all the corpuscles are limited to roatating in a ring. Moving from ring to sphere proves too difficult a challenge.

Here is a quote from the 1904 article:

We suppose that the atom consists of a number of corpuscles moving about in a sphere of uniform positive electrification . . . .

That seems pretty straighforward, but the problem will soon become the electrons and their mass.

By the way, this is often referred to as Thomson's "plum pudding model," where the pudding represents the sphere of positive electricity and the bits of plum scattered in the pudding are the electrons. The ChemTeam likes to call it the "blueberry muffin" model. All those round little blueberries surrounded by the bread of the muffin. Ummmm, good. Some butter on top of a muffin hot from the oven and some nice, COLD milk. Oh my.

However, not everyone is convinced this is the right answer. Savante Arrhenius (the 1903 Nobel Prize winner in Chemistry) had this to say about Thomson's model in 1907:

"This conception has hitherto remained only a formal one, and has led to no new results."

Before leaving this topic, I want to make a point about how the Thomson Model is presented today. Sometimes teachers, and even textbooks, will represent the Thomson Model as a mixture of protons and electrons, like on the right-hand side of this image:

Make sure you have the correct idea firmly in mind. The Thomson Model has negative partices (electrons) and a sphere of positive charge. There are NO protons in the Thomson Model of the atoms. Be careful, a teacher might try to trip you up on a test question. (Those teachers sure are evil, aren't they??)

Interest in the Thomson Model fell off rapidly after 1911, although in 1914 and 1915 attempts were made to resurrect it. These efforts came to nothing and the Thomson Model assumed its place in history as the first modern attempt to construct a theory of atomic structure.