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John Dalton
Born	(1766-09-06)6 September 1766 Eaglesfield, Cumberland, England
Died	27 July 1844(1844-07-27) (aged 77) Manchester, England
Notable students	James Prescott Joule
Known for	Atomic Theory, Law of Multiple Proportions, Dalton's Law of Partial Pressures, Daltonism
Influences	John Gough
Signature

John Dalton FRS (6 September 1766 – 27 July 1844) was an English chemist, meteorologist and physicist. He is best known for his pioneering work in the development of modern atomic theory, and his research into colour blindness (sometimes referred to as Daltonism, in his honour).

Early life[link]

John Dalton was born into a Quaker family at Eaglesfield, near Cockermouth, Cumberland, England. The son of a weaver, he joined his older brother Jonathan at age 15 in running a Quaker school in nearby Kendal. Around 1790 Dalton seems to have considered taking up law or medicine, but his projects were not met with encouragement from his relatives – Dissenters were barred from attending or teaching at English universities – and he remained at Kendal until, in the spring of 1793, he moved to Manchester. Mainly through John Gough, a blind philosopher and polymath to whose informal instruction he owed much of his scientific knowledge, Dalton was appointed teacher of mathematics and natural philosophy at the "New College" in Manchester, a dissenting academy. He remained in that position until 1800, when the college's worsening financial situation led him to resign his post and begin a new career in Manchester as a private tutor for mathematics and natural philosophy.

Dalton's early life was highly influenced by a prominent Eaglesfield Quaker named Elihu Robinson,^[1] a competent meteorologist and instrument maker, who got him interested in problems of mathematics and meteorology. During his years in Kendal, Dalton contributed solutions of problems and questions on various subjects to the Gentlemen's and Ladies' Diaries, and in 1787 he began to keep a meteorological diary in which, during the succeeding 57 years, he entered more than 200,000 observations.^[2] He also rediscovered George Hadley's theory of atmospheric circulation (now known as the Hadley cell) around this time.^[3] Dalton's first publication was Meteorological Observations and Essays (1793), which contained the seeds of several of his later discoveries. However, in spite of the originality of his treatment, little attention was paid to them by other scholars. A second work by Dalton, Elements of English Grammar, was published in 1801.

Colour blindness[link]

This image shows a number 44 or 49, but someone who is deuteranopic may not be able to see it.

In 1794, shortly after his arrival in Manchester, Dalton was elected a member of the Manchester Literary and Philosophical Society, the "Lit & Phil", and a few weeks later he communicated his first paper on "Extraordinary facts relating to the vision of colours", in which he postulated that shortage in colour perception was caused by discoloration of the liquid medium of the eyeball. In fact, a shortage of colour perception in some people had not even been formally described or officially noticed until Dalton wrote about his own.^[4]

Although Dalton's theory lost credence in his own lifetime, the thorough and methodical nature of his research into his own visual problem was so broadly recognized that Daltonism became a common term for colour blindness. Examination of his preserved eyeball in 1995 demonstrated that Dalton actually had a less common kind of colour blindness, deuteroanopia, in which medium wavelength sensitive cones are missing (rather than functioning with a mutated form of their pigment, as in the most common type of colour blindness, deuteroanomaly).^[4] Besides the blue and purple of the spectrum he was able to recognize only one colour, yellow, or, as he says in his paper,

that part of the image which others call red appears to me little more than a shade or defect of light. After that the orange, yellow and green seem one colour which descends pretty uniformly from an intense to a rare yellow, making what I should call different shades of yellow

This paper was followed by many others on diverse topics on rain and dew and the origin of springs, on heat, the colour of the sky, steam, the auxiliary verbs and participles of the English language and the reflection and refraction of light.

Atomic theory[link]

In 1800, Dalton became a secretary of the Manchester Literary and Philosophical Society, and in the following year he orally presented an important series of papers, entitled "Experimental Essays" on the constitution of mixed gases; on the pressure of steam and other vapours at different temperatures, both in a vacuum and in air; on evaporation; and on the thermal expansion of gases. These four essays were published in the Memoirs of the Lit & Phil in 1802.

The second of these essays opens with the striking remark,

There can scarcely be a doubt entertained respecting the reducibility of all elastic fluids of whatever kind, into liquids; and we ought not to despair of effecting it in low temperatures and by strong pressures exerted upon the unmixed gases further.

After describing experiments to ascertain the pressure of steam at various points between 0 and 100 °C (32 and 212 °F), Dalton concluded from observations on the vapour pressure of six different liquids, that the variation of vapour pressure for all liquids is equivalent, for the same variation of temperature, reckoning from vapour of any given pressure.

In the fourth essay he remarks,

I see no sufficient reason why we may not conclude that all elastic fluids under the same pressure expand equally by heat and that for any given expansion of mercury, the corresponding expansion of air is proportionally something less, the higher the temperature. It seems, therefore, that general laws respecting the absolute quantity and the nature of heat are more likely to be derived from elastic fluids than from other substances.

Gas laws[link]

Joseph Louis Gay-Lussac

Jacques Alexandre César Charles, 1820

He thus enunciated Gay-Lussac's law or J.A.C. Charles's law, published in 1802 by Joseph Louis Gay-Lussac. In the two or three years following the reading of these essays, Dalton published several papers on similar topics, that on the absorption of gases by water and other liquids (1803), containing his law of partial pressures now known as Dalton's law.

The most important of all Dalton's investigations are those concerned with the atomic theory in chemistry, with which his name is inseparably associated. It has been proposed that this theory was suggested to him either by researches on ethylene (olefiant gas) and methane (carburetted hydrogen) or by analysis of nitrous oxide (protoxide of azote) and nitrogen dioxide (deutoxide of azote), both views resting on the authority of Thomas Thomson. However, a study of Dalton's own laboratory notebooks, discovered in the rooms of the Lit & Phil,^[5] concluded that so far from Dalton being led by his search for an explanation of the law of multiple proportions to the idea that chemical combination consists in the interaction of atoms of definite and characteristic weight, the idea of atoms arose in his mind as a purely physical concept, forced upon him by study of the physical properties of the atmosphere and other gases. The first published indications of this idea are to be found at the end of his paper on the absorption of gases already mentioned, which was read on 21 October 1803, though not published until 1805. Here he says:

Why does not water admit its bulk of every kind of gas alike? This question I have duly considered, and though I am not able to satisfy myself completely I am nearly persuaded that the circumstance depends on the weight and number of the ultimate particles of the several gases.

Atomic weights[link]

Dalton proceeded to print his first published table of relative atomic weights. Six elements appear in this table, namely hydrogen, oxygen, nitrogen, carbon, sulfur, and phosphorus, with the atom of hydrogen conventionally assumed to weigh 1. Dalton provided no indication in this first paper how he had arrived at these numbers.^{[citation needed]} However, in his laboratory notebook under the date 6 September 1803^[6] there appears a list in which he sets out the relative weights of the atoms of a number of elements, derived from analysis of water, ammonia, carbon dioxide, etc. by chemists of the time.

It appears, then, that confronted with the problem of calculating the relative diameter of the atoms of which, he was convinced, all gases were made, he used the results of chemical analysis. Assisted by the assumption that combination always takes place in the simplest possible way, he thus arrived at the idea that chemical combination takes place between particles of different weights, and it was this which differentiated his theory from the historic speculations of the Greeks, such as Democritus and Lucretius.^{[citation needed]}

The extension of this idea to substances in general necessarily led him to the law of multiple proportions, and the comparison with experiment brilliantly confirmed his deduction.^[7] It may be noted that in a paper on the proportion of the gases or elastic fluids constituting the atmosphere, read by him in November 1802, the law of multiple proportions appears to be anticipated in the words: "The elements of oxygen may combine with a certain portion of nitrous gas or with twice that portion, but with no intermediate quantity", but there is reason to suspect that this sentence may have been added some time after the reading of the paper, which was not published until 1805.

Compounds were listed as binary, ternary, quaternary, etc. (molecules composed of two, three, four, etc. atoms) in the New System of Chemical Philosophy depending on the number of atoms a compound had in its simplest, empirical form.

He hypothesized the structure of compounds can be represented in whole number ratios. So, one atom of element X combining with one atom of element Y is a binary compound. Furthermore, one atom of element X combining with two elements of Y or vice versa, is a ternary compound. Many of the first compounds listed in the New System of Chemical Philosophy correspond to modern views, although many others do not.

Various atoms and molecules as depicted in John Dalton's A New System of Chemical Philosophy (1808).

Dalton used his own symbols to visually represent the atomic structure of compounds. These have made it in New System of Chemical Philosophy where Dalton listed a number of elements, and common compounds.

Five main points of Dalton's atomic theory[link]

1. Elements are made of extremely small particles called atoms.
2. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.
3. Atoms cannot be subdivided, created, or destroyed.
4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
5. In chemical reactions, atoms are combined, separated, or rearranged.

Dalton proposed an additional "rule of greatest simplicity" that created controversy, since it could not be independently confirmed.

When atoms combine in only one ratio, "..it must be presumed to be a binary one, unless some cause appear to the contrary".

This was merely an assumption, derived from faith in the simplicity of nature. No evidence was then available to scientists to deduce how many atoms of each element combine to form compound molecules. But this or some other such rule was absolutely necessary to any incipient theory, since one needed an assumed molecular formula in order to calculate relative atomic weights. In any case, Dalton's "rule of greatest simplicity" caused him to assume that the formula for water was OH and ammonia was NH, quite different from our modern understanding.

Despite the uncertainty at the heart of Dalton's atomic theory, the principles of the theory survived. To be sure, the conviction that atoms cannot be subdivided, created, or destroyed into smaller particles when they are combined, separated, or rearranged in chemical reactions is inconsistent with the existence of nuclear fusion and nuclear fission, but such processes are nuclear reactions and not chemical reactions. In addition, the idea that all atoms of a given element are identical in their physical and chemical properties is not precisely true, as we now know that different isotopes of an element have slightly varying weights. However, Dalton had created a theory of immense power and importance. Indeed, Dalton's innovation was fully as important for the future of the science as Antoine Laurent Lavoisier's oxygen-based chemistry had been.

Later years[link]

James Prescott Joule

Dalton communicated his atomic theory to Thomson who, by consent, included an outline of it in the third edition of his System of Chemistry (1807), and Dalton gave a further account of it in the first part of the first volume of his New System of Chemical Philosophy (1808). The second part of this volume appeared in 1810, but the first part of the second volume was not issued till 1827. This delay is not explained by any excess of care in preparation, for much of the matter was out of date and the appendix giving the author's latest views is the only portion of special interest. The second part of vol. ii. never appeared. For Rees's Cyclopaedia Dalton contributed articles on Chemistry and Meteorology, but the topics are not known.

He was president of the Lit & Phil from 1817 until his death, contributing 116 memoirs. Of these the earlier are the most important. In one of them, read in 1814, he explains the principles of volumetric analysis, in which he was one of the earliest workers. In 1840 a paper on the phosphates and arsenates, often regarded as a weaker work, was refused by the Royal Society, and he was so incensed that he published it himself. He took the same course soon afterwards with four other papers, two of which (On the quantity of acids, bases and salts in different varieties of salts and On a new and easy method of analysing sugar) contain his discovery, regarded by him as second in importance only to the atomic theory, that certain anhydrates, when dissolved in water, cause no increase in its volume, his inference being that the salt enters into the pores of the water.

James Prescott Joule was a famous pupil of Dalton.

Dalton's experimental method[link]

Sir Humphry Davy, 1830 engraving based on the painting by Sir Thomas Lawrence (1769–1830)

As an investigator, Dalton was often content with rough and inaccurate instruments, though better ones were obtainable. Sir Humphry Davy described him as "a very coarse experimenter", who almost always found the results he required, trusting to his head rather than his hands. On the other hand, historians who have replicated some of his crucial experiments have confirmed Dalton's skill and precision.

In the preface to the second part of Volume I of his New System, he says he had so often been misled by taking for granted the results of others that he determined to write "as little as possible but what I can attest by my own experience", but this independence he carried so far that it sometimes resembled lack of receptivity. Thus he distrusted, and probably never fully accepted, Gay-Lussac's conclusions as to the combining volumes of gases. He held unconventional views on chlorine. Even after its elementary character had been settled by Davy, he persisted in using the atomic weights he himself had adopted, even when they had been superseded by the more accurate determinations of other chemists. He always objected to the chemical notation devised by Jöns Jakob Berzelius, although most thought that it was much simpler and more convenient than his own cumbersome system of circular symbols.

Public and personal life[link]

Before he had propounded the atomic theory, he had already attained a considerable scientific reputation. In 1804, he was chosen to give a course of lectures on natural philosophy at the Royal Institution in London, where he delivered another course in 1809–1810. However, some witnesses reported that he was deficient in the qualities that make an attractive lecturer, being harsh and indistinct in voice, ineffective in the treatment of his subject, and singularly wanting in the language and power of illustration.

In 1810, Sir Humphry Davy asked him to offer himself as a candidate for the fellowship of the Royal Society, but Dalton declined, possibly for financial reasons. However, in 1822 he was proposed without his knowledge, and on election paid the usual fee. Six years previously he had been made a corresponding member of the French Académie des Sciences, and in 1830 he was elected as one of its eight foreign associates in place of Davy. In 1833, Earl Grey's government conferred on him a pension of £150, raised in 1836 to £300.

Dalton never married and had only a few close friends, all in all as a Quaker he lived a modest and unassuming life.^[8]

He lived for more than a quarter of a century with his friend the Rev. W. Johns (1771–1845), in George Street, Manchester, where his daily round of laboratory work and tuition was broken only by annual excursions to the Lake District and occasional visits to London. In 1822 he paid a short visit to Paris, where he met many distinguished resident scientists. He attended several of the earlier meetings of the British Association at York, Oxford, Dublin and Bristol.

Death and legacy[link]

Bust of Dalton by Chantrey

Painting portrait of Dalton in later life

Dalton suffered a minor stroke in 1837, and a second one in 1838 left him with a speech impediment, though he remained able to do experiments. In May 1844 he had yet another stroke; on 26 July he recorded with trembling hand his last meteorological observation. On 27 July, in Manchester, Dalton fell from his bed and was found lifeless by his attendant. Approximately 40,000 people filed by his coffin as it was laid in state in the Manchester Town Hall.^[9] He was buried in Manchester in Ardwick cemetery. The cemetery is now a playing field, but pictures of the original grave are in published materials.^[10][11]

A bust of Dalton, by Chantrey, was publicly subscribed for^[12] and placed in the entrance hall of the Royal Manchester Institution. Chantrey also crafted a large statue of Dalton, now in the Manchester Town Hall. The statue was erected while Dalton was still alive and it has been said: "He is probably the only scientist who got a statue in his lifetime".^[9]

In honour of Dalton's work, many chemists and biochemists use the (as yet unofficial) unit dalton (abbreviated Da) to denote one atomic mass unit, or 1/12 the weight of a neutral atom of carbon-12. There is a John Dalton Street connecting Deansgate and Albert Square in the centre of Manchester.

Manchester Metropolitan University has a building named after John Dalton and occupied by the Faculty of Science and Engineering, in which the majority of its Science & Engineering lectures and classes take place. A statue is outside the John Dalton Building of the Manchester Metropolitan University in Chester Street which has been moved from Piccadilly. It was the work of William Theed (after Chantrey) and is dated 1855 (it was in Piccadilly until 1966).

The University of Manchester has a hall of residence called Dalton Hall; it also established two Dalton Chemical Scholarships, two Dalton Mathematical Scholarships, and a Dalton Prize for Natural History. There is a Dalton Medal awarded occasionally by the Manchester Literary and Philosophical Society (only 12 times altogether).

Dalton Township in southern Ontario was named for Dalton. It has, since 2001, been absorbed into the City of Kawartha Lakes. However the township name was used in a massive new park: Dalton Digby Wildlands Provincial Park, itself renamed since 2002.

A lunar crater has been named after Dalton. "Daltonism" became a common term for colour blindness and "Daltonien" is the actual French word for "colour blind".

The inorganic section of the UK's Royal Society of Chemistry is named after Dalton (Dalton Division), and the Society's academic journal for inorganic chemistry also bears his name (Dalton Transactions).

The name Dalton can often be heard in the halls of many Quaker schools, for example, one of the school houses in Coram House, the primary sector of Ackworth School, is called Dalton.

Much of his collected work was damaged during the bombing of the Manchester Literary and Philosophical Society on 24 December 1940. This event prompted Isaac Asimov to say, "John Dalton's records, carefully preserved for a century, were destroyed during the World War II bombing of Manchester. It is not only the living who are killed in war". The damaged papers are now in the John Rylands Library having been deposited in the university library by the Society.

See also[link]

Wikimedia Commons has media related to: John Dalton

Wikisource has original works written by or about: John Dalton

• Atomic mass unit (dalton) – the atomic mass unit
• Dalton Minimum – a period of low solar activity
• Daltonism
• Democritus
• Pneumatic chemistry

References[link]

Bibliography[link]

Statue of Dalton in the Manchester Town Hall.

• Greenaway, Frank (1966). John Dalton and the Atom. Ithaca, New York: Cornell University Press. 
• Henry, William C. (1854). Memoirs of the Life and Scientific Researches of John Dalton. London: Cavendish Society. http://books.google.com/?id=sWVKAAAAMAAJ&printsec=frontcover. Retrieved 24 December 2007. 
• Hunt, D. M.; Dulai, K. S.; Bowmaker, J. K.; Mollon, J. D. (1995). "The Chemistry of John Dalton's Color Blindness". Science 267 (5200): 984–988. Bibcode 1995Sci...267..984H. DOI:10.1126/science.7863342. PMID 7863342. 
• Lonsdale, Henry (1874). The Worthies of Cumberland: John Dalton. George Routledge and Sons: George. http://books.google.com/?id=pog6AAAAMAAJ&printsec=frontcover. Retrieved 24 December 2007. 
• Millington, John Price (1906). John Dalton. London: J. M. Dent & Company. http://books.google.com/?id=S0cDAAAAYAAJ&pg=PA167. Retrieved 24 December 2007. 
• Patterson, Elizabeth C. (1970). John Dalton and the Atomic Theory. Garden City, New York: Anchor. 
• Rocke, A. J. (2005). "In Search of El Dorado: John Dalton and the Origins of the Atomic Theory". Social Research 72: 125–158. http://findarticles.com/p/articles/mi_m2267/is_1_72/ai_n13807654/pg_1. Retrieved 24 December 2007. 
• Roscoe, Henry E. (1895). John Dalton and the Rise of Modern Chemistry. London: Macmillan. http://books.google.com/?id=kmcSAAAAIAAJ&printsec=frontcover. Retrieved 24 December 2007. 
• Roscoe, Henry E.; Arthur Harden (1896). A New View of the Origin of Dalton's Atomic Theory. London: Macmillan. ISBN 1-4369-2630-0. http://books.google.com/?id=0YwEAAAAYAAJ&printsec=frontcover. Retrieved 24 December 2007. 
• Smith, R. Angus (1856). Memoir of John Dalton and History of the Atomic Theory. London: H. Bailliere. ISBN 1-4021-6437-8. http://books.google.com/?id=ZOsAAAAAYAAJ&pg=PP17. Retrieved 24 December 2007. 
• Smyth, A. L. (1998). John Dalton, 1766–1844: A Bibliography of Works by and About Him, With an Annotated List of His Surviving Apparatus and Personal Effects. ISBN 1-85928-438-8. - Original edition published by Manchester University Press in 1966
• Thackray, Arnold (1972). John Dalton: Critical Assessments of His Life and Science. Harvard University Press. ISBN 0-674-47525-9. 

External links[link]

•  "Dalton, John (1766–1844)". Dictionary of National Biography. London: Smith, Elder & Co. 1885–1900. 
• Dalton, John (1834). Meteorological Observations and Essays (2 ed.). Manchester: Harrison and Crosfield. http://books.google.com/?id=Ot8KAAAAIAAJ&printsec=frontcover. Retrieved 24 December 2007. 
• Dalton, John (1893). Foundations of the Atomic Theory. Edinburgh: William F. Clay. http://books.google.com/?id=V5sEAAAAYAAJ&printsec=frontcover. Retrieved 24 December 2007. – Alembic Club reprint with some of Dalton's papers, along with some by William Hyde Wollaston and Thomas Thomson
• Dalton, John (1808). A new system of chemical philosophy. ISBN 1-153-05671-2. http://www.archive.org/details/newsystemofchemi01daltuoft. Retrieved 8 July 2008. 

Niels Bohr
Born	Niels Henrik David Bohr (1885-10-07)7 October 1885 Copenhagen, Denmark
Died	18 November 1962(1962-11-18) (aged 77) Copenhagen, Denmark
Nationality	Danish
Fields	Physics
Institutions	University of Copenhagen University of Cambridge University of Manchester
Alma mater	University of Copenhagen
Doctoral advisor	Christian Christiansen
Other academic advisors	J. J. Thomson Ernest Rutherford
Doctoral students	Hendrik Anthony Kramers
Known for	Copenhagen interpretation Complementarity Bohr model Sommerfeld–Bohr theory BKS theory Bohr-Einstein debates Bohr magneton
Influences	Ernest Rutherford Harald Høffding
Influenced	Werner Heisenberg Wolfgang Pauli Paul Dirac Lise Meitner Max Delbrück and many others
Notable awards	Nobel Prize in Physics (1922) Franklin Medal (1926)
Signature

Niels Henrik David Bohr (Danish pronunciation: [ˈniːls ˈboɐ̯ˀ]; 7 October 1885 – 18 November 1962)^[1] was a Danish physicist who made foundational contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922.^[2] Bohr mentored and collaborated with many of the top physicists of the century at his institute in Copenhagen. He was part of the British team of physicists working on the Manhattan Project. Bohr married Margrethe Nørlund in 1912, and one of their sons, Aage Bohr, grew up to be an important physicist who in 1975 also received the Nobel Prize. Bohr has been described as one of the most influential scientists of the 20th century.^[3]

Biography[link]

Early years[link]

Bohr was born in Copenhagen, Denmark, in 1885. His father, Christian Bohr, was professor of physiology at the University of Copenhagen (it is his name which is given to the Bohr shift or Bohr effect), while his mother, Ellen Adler Bohr, came from a wealthy Jewish family prominent in Danish banking and parliamentary circles (in 1891, Bohr was baptized a Lutheran, his father's religion).^[4]Despite having a religious background, he later became an atheist.^[5]His brother was Harald Bohr, a mathematician and Olympic footballer who played on the Danish national team. Niels Bohr was a passionate footballer as well, and the two brothers played a number of matches for the Copenhagen-based Akademisk Boldklub, with Niels in goal.^[6][7]

Niels Bohr as a young man. Exact date of photo unknown.

In 1903 Bohr enrolled as an undergraduate at Copenhagen University, initially studying philosophy and mathematics. In 1905, prompted by a gold medal competition sponsored by the Royal Danish Academy of Sciences and Letters, he conducted a series of experiments to examine the properties of surface tension, using his father's laboratory in the university, familiar to him from assisting there since childhood. His essay won the prize, and it was this success that decided Bohr to abandon philosophy and adopt physics.^[8] He continued as a graduate student at the University of Copenhagen, under the physicist Christian Christiansen, receiving his doctorate in 1911.

As a post-doctoral student, Bohr first conducted experiments under J. J. Thomson, of Trinity College, Cambridge and Cavendish Laboratory. In 1912 he met and later joined Ernest Rutherford at Manchester University, where on and off he spent four fruitful years in association with the older physics professor. In 1916, Bohr returned permanently to the University of Copenhagen, where he was appointed to the Chair of Theoretical Physics, a position created especially for him. In 1918 he began efforts to establish the University Institute of Theoretical Physics, which he later directed.

Earlier in 1910 Bohr had met Margrethe Nørlund, sister of the mathematician Niels Erik Nørlund.^[9] They were married in Copenhagen in 1912.^[10] Of their six sons, the oldest died in a boating accident and another died from childhood meningitis. The others went on to lead successful lives, including Aage Bohr, who became a very successful physicist and, like his father, was awarded the Nobel Prize in physics, in 1975. His other sons were Hans Henrik, a physician, Erik, a chemical engineer, and Ernest, a lawyer.^[1]

Physics[link]

In 1922, Bohr was awarded the Nobel Prize in physics "for his services in the investigation of the structure of atoms and of the radiation emanating from them."^[11] The award recognized his early leading work in the emerging field of quantum mechanics.

While at Manchester University, Bohr had adapted Rutherford's nuclear structure to Max Planck's quantum theory and so obtained a model of atomic structure which, with later improvements – mainly as a result of Heisenberg's concepts – remains valid to this day. Bohr published his model of atomic structure in 1913.^[12] Here he introduced the theory of electrons traveling in orbits around the atom's nucleus, the chemical properties of each element being largely determined by the number of electrons in the outer orbits of its atoms.^[13] Bohr also introduced the idea that an electron could drop from a higher-energy orbit to a lower one, in the process emitting a photon (light quantum) of discrete energy. This became a basis for quantum theory.^[14]

Among the international community of nuclear physicists, Bohr came to play the role of convener of discussion groups and lectures, as well as being a mentor and an advisor. With the assistance of the Danish government and the Carlsberg Foundation, he succeeded in founding the Institute of Theoretical Physics in 1921, of which he became director.^[15] Bohr's institute served as a focal point for researchers into Quantum Mechanics and related subjects in the 1920s and 1930s, when most of the world's best known theoretical physicists spent some time in his company. Bohr became widely appreciated as their congenial host and eminent colleague, both at the Institute and at the Foundation's mansion in Carlsberg, where he and his family resided after 1932.^[16]

Niels Bohr and Albert Einstein debating quantum theory. Picture taken by Paul Ehrenfest at his home in Leiden (December 1925).

Bohr also conceived the principle of complementarity: that items could be separately analyzed as having several contradictory properties. For example, physicists currently conclude that light behaves either as a wave or a stream of particles depending on the experimental framework – two apparently mutually exclusive properties – on the basis of this principle.^[17] Bohr found philosophical applications for this daring principle.^[specify] Albert Einstein much preferred the determinism of classical physics over the probabilistic new quantum physics (to which among many others Einstein himself had 'unwittingly' contributed). Philosophical issues that arose from the novel aspects of Quantum Mechanics became widely celebrated subjects of discussion. Einstein and Bohr had good-natured arguments over such issues throughout their lives.^[18] See article Bohr–Einstein debates.

Werner Heisenberg worked as an assistant to Bohr and university lecturer in Copenhagen from 1926 to 1927. It was in Copenhagen, in 1927, that Heisenberg developed his uncertainty principle, while working on the mathematical foundations of quantum mechanics. Heisenberg later became head of the German nuclear energy project. In April 1940, early in World War II, Germany invaded and occupied Denmark. In September 1941, Bohr was visited by Heisenberg in Copenhagen.^[19] See section below.

Manhattan Project[link]

In September 1943, reliable word reached Bohr about his imminent arrest by the German police; the Danish resistance quickly managed to help Bohr and his wife escape by sea to Sweden. Soon after, Bohr was flown in a military aircraft to Britain. There he was introduced to the then-secret atomic bomb project (see Political Activities section below for greater detail). Eventually he was directed to the project's principal location in the United States of America.^[20]

Bohr worked on the Manhattan Project at the top-secret Los Alamos laboratory in New Mexico, where he was known by the name of Nicholas Baker for security reasons.^[21] His role on the project was as the knowledgeable consultant or "father confessor". He often expressed social concern about such weaponry and an eventual nuclear arms race, and is quoted as saying, "That is why I went to America. They didn't need my help in making the atom bomb."^[22]

Bohr believed that atomic secrets should be shared by the international scientific community. After meeting with Bohr, J. Robert Oppenheimer suggested that Bohr visit President Franklin D. Roosevelt to convince him that the Manhattan Project should be shared with the Russians in the hope of speeding up its results. Roosevelt suggested that Bohr return to the United Kingdom to try to win British approval. Winston Churchill disagreed with the idea of openness towards the Russians to the point that he wrote in a letter: "It seems to me Bohr ought to be confined or at any rate made to see that he is very near the edge of mortal crimes."^[23]

Coat of arms

Later years[link]

Following the war Bohr returned to Copenhagen. He continued to advocate the peaceful use of nuclear energy. When awarded the Order of the Elephant by the Danish government, he designed his own coat of arms which featured a taijitu (symbol of yin and yang) and the motto in Latin: contraria sunt complementa: "opposites are complementary".^[24] He died in Copenhagen in 1962 of heart failure.^[25] He is buried in the Assistens Kirkegård in the Nørrebro section of Copenhagen.

Contributions to Physics and Chemistry[link]

• The Bohr model of the atom, the theory that electrons travel in discrete orbits around the atom's nucleus.
• The shell model of the atom, where the chemical properties of an element are determined by the electrons in the outermost orbit.
• The correspondence principle, the basic tool of Old quantum theory.
• The liquid drop model of the atomic nucleus.
• Identified the isotope of uranium that was responsible for slow-neutron fission – ²³⁵U.^[26]
• Much work on the Copenhagen interpretation of quantum mechanics.
• The principle of complementarity: that items could be separately analyzed as having several contradictory properties.

Political activity[link]

Denmark during World War II[link]

As regards the Occupation of Denmark during World War II, and especially the events surrounding the Danish policy of cooperation with Nazi Germany and the treatment of Danish Jews, most Danish archives remained sealed until 1998, many remain sealed to this day, and opinions of Danish historical scholars and politicians on these dark topics remain deeply divided.^[27] So too, scholarly opinions remain divided on the importance of the political activities of Niels Bohr during this period. All sources agree that almost as soon as Hitler had taken power in Germany, Bohr played an active role in rescuing Jewish physicists out of Germany, typically offering them haven in Copenhagen before they could take up permanent residence elsewhere.^[a] As for Sweden during World War II and especially in the autumn of 1943, it was far from certain that they would accept Danish Jews attempting to escape Hitler's deportation order. As related by Bohr's friend Stefan Rozental^[28] and the historian Richard Rhodes,^[29] Bohr was immediately smuggled out of Denmark in order to secure his services for the Manhattan project. But rather than proceeding promptly to the United States, as had been planned for him, on 30 September 1943 Bohr persuaded King Gustav of Sweden to make public Sweden's willingness to provide asylum, on 2 October 1943 Swedish radio broadcast that Sweden was ready to offer asylum, and there followed quickly thereafter the mass rescue of the Danish Jews by their countrymen. Historians are divided not on Bohr's political actions in Sweden, but rather on the implications and impacts of those actions. Some argue that Bohr was among those rescued and therefore could have played no role in facilitating the mass rescue, whereas Rhodes^[29] and others^[a] interpret Bohr's political action in Sweden as being a decisive event without which that mass rescue could not have occurred. Whether or not the mass rescue could have happened without Bohr's political activity in Sweden, there is no doubt that he did all that he could for his countrymen.

Nuclear weapons[link]

"Tube Alloys" was the code-name for the British nuclear weapon program. British intelligence inquired about Bohr's availability for work or insights of particular value. Bohr's reply made it clear that he could not help. This reply, like his reaction to Heisenberg, made sure that if Gestapo intercepted anything attributed to Bohr it would point to no knowledge regarding nuclear energy as it stood in 1941. This does not exclude the possibility that Bohr privately made calculations going further than his work in 1939 with Wheeler.

The allied powers learned in 1944 that Germany had no atomic weapons. Bohr hoped then that greater atomic armament could be prevented by some new international agreements, before the weapons were used in the war. He also hoped that new contacts could be established between the Western and Russian nuclear scientists, as signs of cooperation between the soon-to-be victorious powers in the war. Bohr's friend Supreme Court Justice Felix Frankfurter informed President Roosevelt about Bohr's opinions, and a meeting between them was organised 26 August 1944. President Roosevelt, however, did not agree with Bohr.^[22] The British Prime Minister Winston Churchill also refused, after a meeting with Bohr 16 May 1944. He disagreed with the idea of any openness towards the Russians to the point that he wrote in a letter: "It seems to me Bohr ought to be confined or at any rate made to see, that he is very near the edge of mortal crimes."^[30]

After leaving Denmark in the dramatic day and night (October 1943) when most Jews were able to escape to Sweden due to exceptional circumstances (see Rescue of the Danish Jews), Bohr was quickly asked again to join the British effort and he was flown to the UK. He was evacuated from Stockholm in 1943 in an unarmed De Havilland Mosquito operated by British Overseas Airways Corporation (BOAC). Passengers on BOAC's Mosquitos were carried in an improvised cabin in the bomb bay. The flight almost ended in tragedy as Bohr did not don his oxygen equipment as instructed and passed out at high altitude. He would have died had not the pilot surmising from Bohr's lack of response to intercom communication that he had lost consciousness, descended to a lower altitude for the remainder of the flight. Bohr's comment was that he had slept like a baby for the entire flight.

As part of the UK team on "Tube Alloys" Bohr went to Los Alamos. Oppenheimer credited Bohr warmly for his guiding help during certain discussions among scientists there. Discreetly, he met President Franklin D. Roosevelt and later Winston Churchill to warn against the perilous perspectives that would follow from separate development of nuclear weapons by several powers rather than some form of controlled sharing of the knowledge, which would spread quickly in any case. Only in the 1950s, after the Soviet Union's first nuclear weapon test, was it possible to create the International Atomic Energy Agency along the lines of Bohr's suggestion.

Open World[link]

Bohr advocated informing the Soviet authorities that the atomic bomb would soon be in use. In 1944 he obtained an audience with Winston Churchill, who became worried about whether Bohr was a security risk.^[31] In 1950 he addressed an 'Open Letter' to the United Nations.^[32]

Bohr and Heisenberg[link]

Relationship[link]

Bohr and Werner Heisenberg enjoyed at first a strong relationship. It began as one between a mentor and his protégé. Bohr had become aware of Heisenberg's talent during a lecture Heisenberg gave in Göttingen in 1922. During the mid-1920s, Heisenberg worked with Bohr at the institute in Copenhagen. Heisenberg, like most of Bohr's assistants, learned Danish. Heisenberg's uncertainty principle was developed during this period, as was Bohr's complementarity principle.

During the 1930s, however, the relationship became constricted due to political developments in Germany. During World War II, Bohr, who was of half-Jewish ancestry, remained for several years in occupied Denmark. Heisenberg in Germany had become head of the German nuclear effort. Heisenberg made a famous visit to Bohr in September 1941. During a private moment outside with Bohr, Heisenberg, it seems, began to address nuclear energy and morality as well as the war. Neither Bohr nor Heisenberg spoke about it in any detail and left few written records of this part of the meeting, when they were alone.^[33] Bohr seems to have reacted by terminating that conversation abruptly while not giving Heisenberg hints in any direction.

While some suggest that the relationship became strained at this meeting, other evidence shows that the level of contact had been reduced considerably for some time already. Heisenberg suggested that the fracture occurred later. In correspondence to his wife, Heisenberg described the final visit of the trip: "Today I was once more, with Weizsäcker, at Bohr's. In many ways this was especially nice, the conversation revolved for a large part of the evening around purely human concerns, Bohr was reading aloud, I played a Mozart Sonata (A-Major)."^[34] Ivan Supek, one of Heisenberg's students and friends, claimed that the main figure of the meeting was actually Weizsäcker who tried to persuade Bohr to mediate peace between Great Britain and Germany.^[35]

Speculation[link]

In 1957, while the author Robert Jungk was working on the book Brighter Than a Thousand Suns (see letter page 100 in this book), Heisenberg wrote to Jungk explaining that he had visited Copenhagen to communicate to Bohr the views of the German scientists, that production of an atomic weapon was possible with great efforts and this raised enormous responsibilities on the worlds scientists on either side. But Bohr was shocked by the news, that Heisenberg was involved in such studies. And the conversation went wrong because of this. The German attempts were mostly focused on energy production and Heisenberg's circle of colleagues tried probably to keep it that way.^[36] Heisenberg acknowledged that his cryptic approach of the subject had so alarmed Bohr that the discussion failed. Heisenberg nuanced his claims and avoided the implication that he and his colleagues had sabotaged the bomb effort. The production of the bomb in USA showed that it craved enormous resources, which was never considered in this scale in Germany during the war.

When Bohr saw Jungk's erroneous depiction in the Danish translation of the book, he disagreed. He drafted (but never sent) a letter to Heisenberg, stating that he never understood the purpose of Heisenberg's visit and was shocked by Heisenberg's opinion that Germany would win the war. And atomic weapons could be decisive in a long war. Bohr had described the possibility of an atomic weapon in a lecture in England 1939.^[37]

Michael Frayn's play Copenhagen (1998), which was performed in London (for five years), Copenhagen, Gothenburg, Rome, Athens, and Geneva and on Broadway in New York, explores what might have happened at the 1941 meeting between Heisenberg and Bohr. Frayn points in particular to the onus of being one of the few to understand what it would mean to create a nuclear weapon.

Kierkegaard's influence on Bohr[link]

It is generally accepted that Bohr read the 19th century Danish Christian existentialist philosopher, Søren Kierkegaard. Richard Rhodes argues in The Making of the Atomic Bomb^[38] that Bohr was influenced by Kierkegaard via the philosopher Harald Høffding, who was strongly influenced by Kierkegaard and who was an old friend of Bohr's father. In 1909, Bohr sent his brother Kierkegaard's Stages on Life's Way as a birthday gift. In the enclosed letter, Bohr wrote, "It is the only thing I have to send home; but I do not believe that it would be very easy to find anything better.... I even think it is one of the most delightful things I have ever read." Bohr enjoyed Kierkegaard's language and literary style, but mentioned that he had some "disagreement with [Kierkegaard's ideas]."^[39]

Given this, there has been some dispute over whether Kierkegaard influenced Bohr's philosophy and science. David Favrholdt^[40] argues that Kierkegaard had minimal influence over Bohr's work, taking Bohr's statement about disagreeing with Kierkegaard at face value, while Jan Faye^[41] endorses the opposing point of view by arguing that one can disagree with the content of a theory while accepting its general premises and structure.^[42]

Legacy[link]

The Bohr family grave in Assistens kirkegård in Copenhangen, where visitors have begun putting pairs of dice on the base of the monument, presumably with reference to the Bohr–Einstein debates.

• He was one of the founding fathers of CERN in 1954.^[43]
• Received the first ever Atoms for Peace Award in 1957.
• In 1965, three years after Bohr's death, the Institute of Physics at the University of Copenhagen changed its name to the Niels Bohr Institute.
• The Bohr models semicentennial was commemorated in Denmark on 21 November 1963 with a postage stamp depicting Bohr, the hydrogen atom and the formula for the difference of any two hydrogen energy levels: Failed to parse (Missing texvc executable; please see math/README to configure.): h\nu = \epsilon_{2} - \epsilon_{1}\,

.

• Bohrium (a chemical element, atomic number 107) is named in honour of Bohr.
• Hafnium, another chemical element, whose properties were predicted by Bohr, was named by him after Hafnia, Copenhagen's Latin name.
• Asteroid 3948 Bohr is named after him.
• The Centennial of Bohr's birth was commemorated in Denmark on 3 October 1985 with a postage stamp depicting Bohr with his wife Margrethe.
• In 1997 the Danish National Bank started circulating the 500-krone banknote with the portrait of Bohr smoking a pipe.^[44]

See also[link]

• Atomic physics
• Bohr-Einstein debates
• Bohr radius
• History of quantum mechanics
• Niels Bohr International Gold Medal
• Standard Model
• List of Jewish Nobel laureates

Reference notes[link]