The bohr model of the atom
Henry Moseley (1887-1915): A British chemist, Henry Moseley studied under Rutherford and
brilliantly developed the application of X-ray spectra to study atomic structure; Moseley's
discoveries resulted in a more accurate positioning of elements in the Periodic Table by closer
determination of atomic numbers. Tragically for the development of science, Moseley was killed
in action at Gallipoli in 1915.
In 1913, almost fifty years after Mendeleev, Henry Moseley published the results of his
measurements of the wavelengths of the X-ray spectral lines of a number of elements which
showed that the ordering of the wavelengths of the X-ray emissions of the elements coincided
with the ordering of the elements by atomic number. With the discovery of isotopes of the
elements, it became apparent that atomic weight was not the significant player in the periodic law
as Mendeleev, Meyers and others had proposed, but rather, the properties of the elements varied
periodically with atomic number.
When atoms were arranged according to increasing atomic number, the few problems with
Mendeleev's periodic table had disappeared. Because of Moseley's work, the modern periodic
table is based on the atomic numbers of the elements.
Rutherford, Ernest (1871-1937): Born in New Zealand, Rutherford studied under J. J. Thomson at
the Cavendish Laboratory in England. His work constituted a notable landmark in the history of
atomic research as he developed Bacquerel's discovery of Radioactivity into an exact and
documented proof that the atoms of the heavier elements, which had been thought to be
immutable, actually disintegrate (decay) into various forms of radiation.
Rutherford was the first to establish the theory of the nuclear atom and to carry out a
transmutation reaction (1919) (formation of hydrogen and and oxygen isotope by bombardment of
nitrogen with alpha particles). Uranium emanations were shown to consist of three types of rays,
alpha (helium nuclei) of low penetrating power, beta (electrons), and gamma, of exceedingly short
wavelength and great energy.
Ernest Rutherford also discovered the half-life of radioactive elements and applied this to studies
of age determination of rocks by measuring the decay period of radium to lead-206
The electron cloud model was invented throughout the 20th century. (1900's)
Born to Wealth. Antoine-Laurent Lavoisier was born August 26, 1743, the son of a
wealthy Paris family. His father was a lawyer who had married a daughter of the wealthy
Punctis family. Louis XV was the King of France. Most of Europe, and especially
France, was in social upheaval. Peasants faced continual famines and peasant revolts and
mob violence were common. Lavoisier's family were among the upper class so Lavoisier
was able to complete a degree in law at the Collège Mazarin in fulfillment of his family's
wishes.
Science. Lavoisier never practiced law. At age 21 began to fulfill his own dream � to
study mathematics and science. He studied astronomy, botany and geology under
eminent scientists of the time. His work with geology and his winning essay on the best
means of lighting the streets of a large city at night gained him an elected membership at
the age of 25 into France's prestigious Academy of Sciences.
Ferme Générale. In 1768, Priestley bought into the Ferme Générale, a private company
that collected taxes for the Crown. Owners, called 'tax farmers,' were empowered to
collect taxes of all kinds, but especially duties on imported goods. The system was easily
and often abused by the tax farmers who enriched themselves and lived in extravagance.
They were the target of popular hatred among the peasants and merchants alike. All
evidence suggests that Lavoisier discharged his duties honestly and without corruption.
Lavoisier seems to have justified his involvement in the Ferme to raise money for the
pursuit of science.
Marie-Anne Pierrette Paulze. In 1771, Lavoisier married 13-year old Marie-Anne
Pierette Paulze, the daughter of a co-owner of the Ferme. With time, she proved to be a
scientific colleague to her husband by learning English so she could translate the writings
of Priestley and others and by developing skills in art and engraving. Madame Lavoisier
drew the sketches of Lavoisier's apparatuses and laboratory, including all of the drawings
in his book, Traité élémentaire de chemie. She developed a scientific mind and was
known to take lively part in discussions of writings on phlogiston and the chemical
results of others.
Chemical Revolution. Over the 20 year period 1770 - 1790, the science of chemistry
experienced a revolution so fundamental and so complete that there has been nothing like
it since. The architect of the revolution was one man � Antoine Lavoisier. Lavoisier
believed that weight was conserved through the course of chemical reactions � even
those involving gases. He explained combustion (and respiration) in terms of chemical
reactions that involve a component of air which he called oxygen. His venue for the
chemical revolution came in 1775, when he was appointed Commissioner of the Royal
Gunpowder and Saltpeter Administration. As such, he was able to build a fine laboratory
at the Paris Arsenal and make important connections to the scientific community of all of
Europe. One of the first chemists to adopt Lavoisier's theories was Joseph Black who
taught them as early as 1784.
Oxygen and the end of Phlogiston. In 1774, Lavoisier was repeating Robert Boyle's tin
calx experiments from the previous century. Boyle knew the tin gained weight as the
calx was formed. The doctrine of Phlogiston explained that phlogiston was released upon
calx formation. While modern scientists recognize the implication that this: phlogiston
must have a negative weight, early phlogistonists (Becker, Stahl) were not bothered as
they considered phlogiston to be something of a philosophical concept. Later
phlogistonists such as Priestley did consider phlogiston to be a material substance
(Cavendish believed it to be his inflammable air, now H2) but because the theory
explained so many chemical phenomena, they were able to overlook its shortcomings.
But not Lavoisier!
Lavoisier heated tin in air in a closed vessel. The tin increased in mass upon forming
the calx [now SnO] and air rushed into the vessel as it was opened. At about the same
time, in October, 1774 Priestley visited Paris and met with Lavoisier and told him at a
dinner party of his discovery of dephlogisticated air. By coincidence, Lavoisier also
received a letter from Scheele (dated September 30, 1774) asking him to repeat one of his
experiments that produced [oxygen]. In November 1774, Lavoisier repeated Priestley's
experiment.
By this time Lavoisier was an affirmed anti-phlogistonist. In 1777, Lavoisier
conducted an experiment that established a fatal shortcoming of the phlogiston theory.
He heated mercury and air using a bell-jar for 12 days. Red mercury calx (now HgO)
formed and the volume of air decreased from 50 to 42 in3. The remaining air was
determined to be atmospheric mofette, and later renamed azote (now nitrogen). The red
[HgO] was heated in a retort producing 8 in3 of dephlogisticated air [O2]. The sequence
of experiments established that heat caused formation of a calx (the doctrine of
phlogiston explained phlogiston was released):
Hg(l) + O2(g)
HgO(s)
and then stronger heating reverted the calx back to the original substances (which the
doctrine of phlogiston would predict to be impossible):
HgO(s)
Hg(l) + O2(g)
Water. Proof of the validity of Lavoisier's Oxygen Theory came when Lavoisier (a)
decomposed water into two gases, which he named hydrogen and oxygen, and then (b)
reformed them into water as had been previously done by Priestley (1781) and then
quantitatively by Cavendish.
Traité élémentaire de chemie, 1789. To spread his ideas and the Oxygen Theory,
Lavoisier published Traité élémentaire de chemie in 1789. In his book, Lavoisier named
a total of 33 elements, most of which are still in use today. It has been said that the book
would be recognizable to a student of chemistry as is reads "like a rather old edition of a
modern textbook."1
French Revolution. Lavoisier was a political liberal. Through the events that led up to
the French Revolution, Lavoisier contributed to the plans for reform � including the
establishment of the metric system. After the French Revolution, Lavoisier was a
member of the Commission for the Establishment of the Metric System and was
appointed Secretary of the Treasury in 1791.
A theorist. Antoine-Laurent Lavoisier discovered no new substances. He made few new
improvements to laboratory methods. Yet he will be remembered to the end of time as
the father of modern chemistry. He took the works of others, most notably that of
Priestley, Black, Cavendish and Scheele and explained it.
"Lavoisier, though a great architect in the science, labored little in the quarry; his
materials were chiefly shaped to his hand, and his skill was displayed in their
arrangement and combination."2
Guillotined! Despite all of the contributions to science and France made by Lavoisier in
his 51-year life, it was his connection with the Ferme Générale that the revolution zealots
noted. In November 1793, all 32 former members of the Ferme Générale were arrested
and imprisoned. After a trial by jury, Antoine-Laurent Lavoisier, along with his fatherin-law and, were found guilty of conspiracy against the people of France. He was
guillotined on May 8, 1794.
"A moment was all that was necessary in which to strike off this head, and probably a
hundred years will not be sufficient to produce another like it."3
Lasting accomplishments.
� father of modern chemistry � the concept of the chemical element
� metric system (replaced hundreds of archaic systems in use throughout France)
� other accomplishments are dwarfed by Lavoisier's oxygen theory which has changed
the course of chemistry forever, but it need be mentioned that Lavoisier is also credited
with other accomplishments, including the invention of plaster of Paris in 1768.
This article is about the structure of an atom. For the particle accelerator
phenomenon, see Electron-Cloud Effect.
Electron cloud is a term used, if not originally coined, by the Nobel Prize laureate and
acclaimed educator Richard Feynman in The Feynman Lectures on Physics for
discussing "exactly what is an electron?". This intuitive model provides a simplified way
of visualizing an electron as a solution of the Schrödinger equation. In the electron cloud
analogy, the probability density of an electron, or wavefunction, is described as a small
cloud moving around the atomic or molecular nucleus, with the opacity of the cloud
proportional to the probability density.
The model evolved from the earlier Bohr model, which likened an electron orbiting an
atomic nucleus to a planet orbiting the sun. The electron cloud formulation better
describes many observed phenomena, including the double slit experiment, the periodic
table and chemical bonding, and atomic interactions with light. Although lacking in
certain details, the intuitive model roughly predicts the experimentally observed waveparticle duality, in that electron behavior is described as a delocalized wavelike object,
yet compact enough to be considered a particle on certain length-scales.
Experimental evidence suggests that the probability density is not just a theoretical model
for the uncertainty in the location of the electron, but rather that it reflects the actual state
of the electron. This carries an enormous philosophical implication, indicating that pointlike particles do not actually exist, and that the universe's evolution may be
fundamentally uncertain. The fundamental source of quantum uncertainty is an unsolved
problem in physics.
In the electron cloud model, rather than following fixed orbits, electrons bound to an
atom are observed more frequently in certain areas around the nucleus called orbitals.
The electron cloud can transition between electron orbital states, and each state has a
characteristic shape and energy, all predicted by the Schrödinger equation, which has
infinitely many solutions. Experimental results motivated this conceptual refinement of
the Bohr model. The famous double slit experiment demonstrates the random behavior of
electrons, as free electrons shot through a double slit are observed at random locations at
a screen, consistent with wavelike interference. Heisenberg's uncertainty principle
accounts for this and, taken together with the double slit experiment, implies that an
electron behaves like a spread of infinitesimal pieces, or "cloud", each piece moving
somewhat independently as in a churning cloud. These pieces can be forced to coincide at
an isolated point in time, but then they all must move relative to each other at an
increased spread of rates to conserve the "uncertainty". Certain physical interactions of
this wavelike electron, such as observing which slit an electron passes through in the
double-slit experiment, require this coincidence of pieces into a lump-like particle. In
such an interaction the electron "materializes", "lumps", or "is observed" at the location
of one of the infinitesimal pieces, apparently randomly chosen. Although the cloud
shrinks to the accuracy of the observation (if observed by light for example the
wavelength of the light limits the accuracy), its momentum spread increases so that
Heisenberg's uncertainty principle is still valid.
Unlike the fixed orbit conceptualization, an electron cloud bound in an atom is not
predicted to collapse towards the charge nucleus, while emitting photons, in order to
minimize the sum of electric potential and kinetic energies, since the "cloud" would gain
too much kinetic energy, as required to conserve uncertainty. The smear obeys
Schrödinger's equation (see also Erwin Schrödinger), which has discrete solutions at
differing energy levels. These solutions are often depicted with density scatter plots or
grayscale maps, which resemble a cloud. This predicts light interactions with an atom, as
electrons transition between these cloud states by absorbing or emitting photons
equivalent to the difference, or quantum, in their energy. Also, the periodic table is
predicted as an electron is added to the lowest unoccupied energy orbital in progressing
from hydrogen to helium, and to subsequent elements, with properties that match those
predicted by the orbital solutions to Schrödinger's equation.
The term "electron cloud" carries specific connotations in atomic and particle physics,
where everyday experience does not extrapolate well. Additional experiments, such as
the behavior of electrons in high speed accelerators, have resulted in more sophisticated
models including quantum electrodynamics and quantum field theory. However, what
drives the uncertainty in the electron cloud model remains one of the great
mysteries of physics
3. Plumb Pudding Model (1897)- Joseph John
Thomson proposed that the atom was a sphere of
positive electricity (which was diffuse) with negative
particles imbedded throughout after discovering the
electron, a discovery for which he was awarded the
Nobel Prize in physics in 1906
Thompson model
4. Solar System Model-Ernest Rutherford discovered
that the atom is mostly empty space with a dense
positively charged nucleus surrounded by negative
electrons. Rutherford received the Nobel Prize in
chemistry in 1908 for his contributions into the
structure of the atom. In 1913 Neils Bohr proposed
that electrons traveled in circular orbits and that only
certain orbits were allowed. This model of the atom
helped explain the emission spectrum of the hydrogen
atom. He received the Nobel Prize in physics in 1922
for his theory.
Erwin Schrödinger built upon the thoughts of Bohr yet took them in a new
direction. He developed the probability function for the Hydrogen atom (and a
few others). The probability function basically describes a cloud-like region
where the electron is likely to be found. It can not say with any certainty,
where the electron actually is at any point in time, yet can describe where it
ought to be. Clarity through fuzziness, is one way to describe the idea. The
model based on this probability equation can best be described as the cloud
model.
The cloud model represents a sort of history of where the electron has probably
been and where it is likely to be going. The red dot in the middle represents the
nucleus while the red dot around the outside represents an instance of the
electron. Imagine, as the electron moves it leaves a trace of where it was. This
collection of traces quickly begins to resemble a cloud. The probable locations
of the electron predicted by Schrödinger's equation happen to coincide with the
locations specified in Bohr's model.
Johannes Wilhelm Geiger was born in Neustadt an-der-Haardt (now
Neustadt an-der-Weinstrasse), Rhineland-Palatinate, Germany, on
September 30, 1882. His father, Wilhelm Ludwig Geiger, was a professor
of philology at the University of Erlangen from 1891 to 1920. The eldest
of five children, two boys and three girls, Geiger was educated initially at
Erlangen Gymnasium, from which he graduated in 1901. After completing
his compulsory military service, he studied physics at the University of
Munich, and at the University of Erlangen where his tutor was Professor
Eilhard Wiedemann. He received a doctorate from the latter institution in
1906 for his thesis on electrical discharges through gases.
Joins Ernest Rutherford in Manchester
That same year, Geiger moved to Manchester University in England to
join its esteemed physics department. At first he was an assistant to its
head, Arthur Schuster, an expert on gas ionization. When Schuster
departed in 1907, Geiger continued his research with Schuster's
successor, Ernest Rutherford, and the young physicist Ernest Marsden.
Rutherford was to have a profound influence on young Geiger, sparking
his interest in nuclear physics. Their relationship, which began as partners
on some of Geiger's most important experiments, was lifelong and is
documented in a series of letters between them.
In addition to supervising the research students working at the lab,
Geiger began a series of experiments with Rutherford on radioactive
emissions, based on Rutherford's detection of the emission of alpha
particles from radioactive substances. Together they began researching
these alpha particles, discovering among other things that two alpha
particles appeared to be released when uranium disintegrated. Since
alpha particles can penetrate through thin walls of solids, Rutherford and
Geiger presumed that they could move straight through atoms. Geiger
designed the apparatus that they used to shoot streams of alpha particles
through gold foil and onto a screen where they were observed as
scintillations, or tiny flashes of light.
Manually counting the thousands of scintillations produced per minute
was a laborious task. Geiger was reputedly something of a workaholic,
who put in long hours recording the light flashes. David Wilson noted in
Rutherford: Simple Genius that in a 1908 letter to his friend Henry A.
Bumstead, Rutherford remarked, "Geiger is a good man and work[s] like
a slave.... [He] is a demon at the work and could count at intervals for a
whole night without disturbing his equanimity. I damned vigorously after
two minutes and retired from the conflict." Geiger was challenged by the
haphazardness of their methodology to invent a more precise technique.
His solution was a primitive version of the "Geiger counter," the machine
with which his name is most often associated. This prototype was
essentially a highly sensitive electrical device designed to count alpha
particle emissions.
Geiger's simple but ingenious measuring device enabled him and
Rutherford to discern that alpha particles are, in fact, doubly charged
nuclear particles, identical to the nucleus of helium atoms traveling at
high velocity. The pair also established the basic unit of electrical charge
when it is involved in electrical activity, which is equivalent to that carried
by a single hydrogen atom. These results were published in two joint
papers in 1908 entitled "An Electrical Method of Counting the Number of
Alpha Particles" and "The Charge and Nature of the Alpha Particle."
In bombarding the gold with the alpha particles Geiger and Rutherford
observed that the majority of the particles went straight through.
However, they unexpectedly found that a few of the particles were
deflected or scattered upon contact with the atoms in the gold, indicating
that they had come into contact with a very powerful electrical field.
Rutherford's description of the event as recorded by Wilson revealed its
importance: "It was as though you had fired a fifteen-inch shell at a piece
of tissue paper and it had bounced back and hit you." These observations
were jointly published by Geiger and Marsden in an article entitled "On a
Diffuse Reflection of the Alpha-Particles" for the Proceedings of the Royal
Society in June of 1909.
Thirty years later Geiger recollected, "At first we could not understand
this at all," Wilson noted. Geiger continued to study the scattering effect,
publishing two more papers about it that year. The first, with Rutherford,
was entitled "The Probability Variations in the Distribution of AlphaParticles." The second, referring to his work with Marsden, dealt with "The
Scattering of Alpha-Particles by Matter." Geiger's work with Rutherford
and Marsden finally inspired Rutherford in 1910 to conclude that the
atoms contained a positively charged core or nucleus which repelled the
alpha particles. Wilson noted Geiger's recollection that "One day
Rutherford, obviously in the best of spirits, came into my [laboratory] and
told me that he now knew what the atom looked like and how to explain
the large deflections of the alpha-particles. On the very same day, I
began an experiment to test the relation expected by Rutherford between
the number of scattered particles and the angle of scattering."
Geiger's results were accurate enough to persuade Rutherford to go
public with his discovery in 1910. Nonetheless, Geiger and Marsden
continued their experiments to test the theory for another year,
completing them in June of 1912. Their results were published in German
in Vienna in 1912 and in English in the Philosophical Magazine in April of
1913. Wilson noted that Dr. T. J. Trenn, a modern physics scholar,
characterized Geiger's and Marsden's work of this period: "It was not the
Geiger-Marsden scattering evidence, as such, that provided massive
support for Rutherford's model of the atom. It was, rather, the
constellation of evidence available gradually from the spring of 1913 and
this, in turn, coupled with a growing conviction, tended to increase the
significance or extrinsic value assigned to the Geiger-Marsden results
beyond that which they intrinsically possessed in July 1912."
In 1912 Geiger gave his name to the Geiger-Nuttal law, which states that
radioactive atoms with short half-lives emit alpha particles at high speed.
He later revised it, and in 1928, a new theory by George Gamow and
other physicists made it redundant. Also in 1912 Geiger returned to
Germany to take up a post as director of the new Laboratory for
Radioactivity at the Physikalisch-Technische Reichsanstalt in Berlin, where
he invented an instrument for measuring not only alpha particles but beta
rays and other types of radiation as well.
Geiger's research was broadened the following year with the arrival at the
laboratory of James Chadwick and Walter Bothe, two distinguished
nuclear physicists. With the latter, Geiger formed what would be a long
and fruitful professional association, investigating various aspects of
radioactive particles together. However, their work was interrupted by the
outbreak of the First World War. Enlisted with the German troops, Geiger
fought as an artillery officer opposite many of his old colleagues from
Manchester including Marsden and H. G. J. Moseley from 1914 to 1918.
The years spent crouching in trenches on the front lines left Geiger with
painful rheumatism. With the war over, Geiger resumed his post at the
Reichsanstalt, where he continued his work with Bothe. In 1920, Geiger
married Elisabeth Heffter, with whom he had three sons.
Perfects the Geiger-Mueller Counter
Geiger moved from the Reichsanstalt in 1925 to become professor of
physics at the University of Kiel. His responsibilities included teaching
students and guiding a sizable research team. He also found time to
develop, with Walther Mueller, the instrument with which his name is
most often associated: the Geiger-Mueller counter, commonly referred to
as the Geiger counter. Electrically detecting and counting alpha particles,
the counter can locate a speeding particle within about one centimeter in
space and to within a hundred-millionth second in time. It consists of a
small metal container with an electrically insulated wire at its heart to
which a potential of about 1000 volts is applied. In 1925, Geiger used his
counter to confirm the Compton effect, that is, the scattering of X rays,
which settled the existence of light quantum, or packets of energy.
Geiger left Kiel for the University of Tubingen in October of 1929 to serve
as professor of physics and director of research at its physics institute.
Installed at the Institute, Geiger worked tirelessly to increase the Geiger
counter's speed and sensitivity. As a result of his efforts, he was able to
discover simultaneous bursts of radiation called cosmic-ray showers, and
concentrated on their study for the remainder of his career.
Geiger returned to Berlin in 1936 upon being offered the chair of physics
at the Technische Hochschule. His upgrading of the counter and his work
on cosmic rays continued. He was also busy leading a team of nuclear
physicists researching artificial radioactivity and the by-products of
nuclear fission (the splitting of the atom's nucleus). Also in 1936 Geiger
took over editorship of the journal Zeitschrift fur Physik, a post he
maintained until his death. It was at this time that Geiger also made a
rare excursion into politics, prompted by the rise to power in Germany of
Adolf Hitler's National Socialist Party. The Nazis sought to harness physics
to their ends and engage the country's scientists in work that would
benefit the Third Reich. Geiger and many other prominent physicists were
appalled by the specter of political interference in their work by the Nazis.
Together with Werner Karl Heisenberg and Max Wien, Geiger composed a
position paper representing the views of most physicists, whether
theoretical, experimental, or technical. As these men were politically
conservative, their decision to oppose the National Socialists was taken
seriously, and seventy-five of Germany's most notable physicists put their
names to the Heisenberg-Wien-Geiger Memorandum. It was presented to
the Reich Education Ministry in late 1936.
The document lamented the state of physics in Germany, claiming that
there were too few up-and-coming physicists and that students were
shying away from the subject because of attacks on theoretical physics in
the newspapers by National Socialists. Theoretical and experimental
physics went hand in hand, it continued, and attacks on either branch
should cease. The Memorandum seemed to put a stop to attacks on
theoretical physics, in the short term at least. It also illustrated how
seriously Geiger and his associates took the threat to their work from the
Nazis.
Geiger continued working at the Technische Hochschule through the war,
although toward the latter part he was increasingly absent, confined to
bed with rheumatism. In 1938 Geiger was awarded the Hughes Medal
from the Royal Academy of Science and the Dudell Medal from the
London Physics Society. He had only just started to show signs of
improvement in his health when his home near Babelsberg was occupied
in June of 1945. Suffering badly, Geiger was forced to flee and seek
refuge in Potsdam, where he died on September 24, 1945.
Sir Ernest Marsden (1889 - 1970), was a British-New Zealand physicist.
Born in Lancashire - 19 February 1889, he met Ernest Rutherford at the
University of Manchester. While still an undergraduate he conducted the
famous alpha particle scattering experiment in 1908 together with Hans
Geiger under Rutherford's supervision.
In 1914 he moved to Victoria University in Wellington, New Zealand.
Marsden served in France during World War I as a Royal Engineer,
earning the Military Cross. Following the war he became New Zealand's
leading scientist, founding the Department of Scientific and Industrial
Research (DSIR) in 1926 and organizing its research particularly in the
area of agriculture. During World War II he worked on radar research,
and in 1947 became scientific liaison officer in London. He died at his
home on Wellington bay in 1970.
Marsden's career recognitions included fellowship in the Royal Society of
London in 1946, president of the Royal Society of New Zealand in 1947,
and knighthood in 1958.