The History of the ATOM

By John DeBoer
Chemisty I History Lesson
October 2008

  • Democritus 460-370 B.C.
  • Plato 427-347 B.C.
  • Aristotle 384-322 B.C.
  • Antoine Lavoisier 1743-1794
  • Charles Coulomb 1736-1806
  • John Dalton 1766-1844
  • William Crookes 1832-1919
  • Wilhelm C. Roentgen 1845-1940
  • Henri Becquerel 1852-1908
  • Marie Curie 1867-1934
  • J.J. Thomson 1856-1940
  • Ernest Rutherford 1871-1937
  • Albert Einstein 1879-1955
  • Robert Millikan 1868-1953
  • Niels Bohr 1885-1962
  • Erwin SchrOdinger 1877-1961
  • Werner Heisenberg 1901-1976
  • James Chadwick 1891-1974
  • Paul Dirac 1902-1984
  • Aage Niels Bohr 1922-

"Democritus (460-370 B.C.)
Year of Discovery- c. 410bc
Country of Origin- Greece


Democritus theorized that matter was ultimately composed of
small, indivisible particles called atomos, or "atoms," meaning

Small, spherical, solid, indivisible model

Indivisible Object

He then suggested that if you divided matter into
smaller and smaller pieces, you would eventually end up
with tiny, indestructible particles--- atoms.

In Democritus' own words:

"By convention there is sweet, by convention there is bitterness, by conventionhot and cold, by convention color; but in reality there are only atoms and the void."

Plato (427-347 B.C.)
Year of Discovery- c. 360bc
Country of Origin- Greece


Plato objected to the idea of
Democritus. He argued that atoms just
crashing into other atoms could never produce the beauty
and form of the world. He postulated the geometric structure
of the simple bodies of the four elements. Since the simple bodies
could be decomposed into triangles, and the triangles
reassembled into atoms of different elements, Plato's
model offered a plausible account of changes among the
primary substances.

Early Cosmology:
The Greeks constructed their whole theory of the Universe on the concepts of numbers and shape, arithmetic and geometry. An example of this was when Euclid discovered that there exist only 5 regular solids. Plato then proposed that these five solids correspond to the four atomic elements in the Universe (earth, water, air and fire) and hypothesized that there existed a five element, called quintessence, which made up the heavenly spheres

Plato proposed these five solids

Aristotle (384-322 B.C.)
Year of Discovery-347-322B.C.
Country of Origin-Greece

Elemental Square
Aristotle explained that in each Element one Power is dominant. Therefore Earth is predominantly Dry, Water predominantly Cool, Air predominantly Moist, and Fire predominantly Warm. The dominant Power is the one in a counterclockwise direction from the Element in the Square The arrow by each Element points to its dominant Power. The vertical axis represents the active Qualities (Warm, Cool), the horizontal represents the passive (Moist, Dry). The upper Elements (Air, Fire) are active, light and ascending, the lower (Water, Earth) are passive, heavy and descending. The Elements on the right are pure, extreme and absolutely light (Fire) or heavy (Earth); those on the left are mixed, intermediate and relatively light (Air) or heavy (Water). The absolute Elements exhibit unidirectional motion (ascending Fire, descending Earth), whereas the relative Elements (Air, Water) can also expand horizontally. The Organic Cycle (the cycle of the seasons) goes sunwise around the square.

Antoine Lavoisier (1743-1794)
Year of Discovery- (1778-1789)
Country of Origin-France
Cover of Antoine's Book

Antoine "PIMP" Lavoisier
French chemist who became the "father of modern chemistry". As a student, he stated "I am young and avid for glory." He was educated. He won a prize on lighting the
streets of Paris, and designed a new method for preparing
saltpeter. He also married a young, beautiful 13-year-old
girl named Marie-Anne, who translated from English for
him and illustrated his books. Lavoisier demonstrated
with careful measurements that transmutation of water
to earth was not possible, but that the sediment observed
from boiling water came from the container. He burnt
phosphorus and sulfur in air, and proved that the products
weighed more than he original. Nevertheless, the weight
gained was lost from the air. He then established the
Law of Conservation of Mass.

This is Antoine Lavoisier's first book. The first phase in a series of experiments which ultimately led the discovery of oxygen. In order to have the book published, Lavoisier had to reperform all the experiments before the Commissioners of the Royal Academy of Sciences in Paris.

first stages in a series of experiments

Charles Coulomb (1736-1806)
Year of Discovery- 1780's
Country of Origin-France

Charles Coulomb
Charles Coulomb
torsion balance
torsion balance

Charles Coulomb is best known for developing Coulomb's law: the definition of the electrostatic force of attraction and repulsion. The SI unit of charge, the coulomb, was named after him. He discovered the law that states "The force between two electrical charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them". It is the main force involved in atomic reactions.

John Dalton (1766-1844)
some of Daltons symbols for the elements

Year of Discovery- 1798
Country of Origin-England

John Dalton looks like D. Reich
The ideas of Democritus were not widely accepted, and it
was not until 1808--- over 2000 years later--- that John Dalton
formalized a theory of atoms that gained broad acceptance.
Dalton's atomic theory has three parts:

1.) each element is composed of tiny indestructible
particles called atoms.

2.) all atoms of a given element have the same mass and
other properties that distinguish them from atoms of other

3.) atoms combine in simple, whole number ratios
to form compounds.

William Crookes (1832-1919)
Year of Discovery- 1870
Country of Origin-England

William Crookes

Sir William Crookes invented the Crookes tube around
Crookes Tube

1875, consisting essentially of a sealed glass tube from
which nearly all the air has been removed and through
the walls of which are passed two electrodes. When
high voltage is applied between the two electrodes,
electrons are emitted from the cathode and are
accelerated toward the anode. Many of these electrons,
or cathode rays miss the anode and strike the wall of the
tube causing it to exhibit fluorescence. The behavior of the
rays indicates that they travel in straight lines and
exert a pressure on any object placed in their path.
The Crookes tube was used by Crookes in a number
of experiments and was later used in experiments leading
to the discovery of X rays by W. C. Roentgen (1895)
and of the electron by J. J. Thomson (1897).

Wilhelm C. Rontgen (1845-1940)
Year of Discovery- 1874
Country of Origin-Germany

Rontgen X-rayed his wifes hand

Wilhelm Conrad Rontgen
On the evening of November 8, 1895, he found that, if the discharge tube is enclosed in a sealed, thick black carton to exclude all light, and if he worked in a dark room, a paper plate covered on one side with barium platinocyanide placed in the path of the rays became fluorescent even when it was as far as two metres from the discharge tube. During subsequent experiments he found that objects of different thicknesses interposed in the path of the rays showed variable transparency to them when recorded on a photographic plate. When he immobilised for some moments the hand of his wife in the path of the rays over a photographic plate, he observed after development of the plate an image of his wife's hand which showed the shadows thrown by the bones of her hand and that of a ring she was wearing, surrounded by the penumbra of the flesh, which was more permeable to the rays and therefore threw a fainter shadow. This was the first "röntgenogram" ever taken. In further experiments, Röntgen showed that the new rays are produced by the impact of cathode rays on a material object. Because their nature was then unknown, he gave them the name X-rays. Later, Max von Laue and his pupils showed that they are of the same electromagnetic nature as light, but differ from it only in the
higher frequency of their vibration. (

Henri Becquerel (1852-1908)
Year of Discovery- 1896
Country of Origin-France
Photographic plate radioactivity

Henri Becquerel
In 1896, Becquerel discovered radioactivity. He hypothesized that x-rays were emitted in conjunction with phosphorescence, the long-lived emission of light that sometimes follows the absorption of light by some atoms and molecules. Becquerel places crystals composed of potassium uranyl sulfate, a compound known as phosphorece-- on top of a photographic plate wrapped in black cloth. He then placed the black cloth and crystals outside and exposed them to light. He knew the crystals had phosphoresced because he could see the emitted light when he brought it back into the dark. Becquerel performed this experiment many times. He later realized that the crystals themselves were constantly emitting something that exposed the photographic plate, independent of whether they phosphoresced. Becquerel concluded that it was the uranium within the crystals and he called the emission uranic rays.

Marie Curie (1867-1934)
Year of Discovery- 1898
Country of Origin-France

Marie Sklodowska Curie
Marie & Pierre Curie

Marie Curie discovered two new elements, both of which
emitted uranic rays. She named them Polonium and
Radium. To do this she and her husband took Pitchblend,
which contained the radioactive element Uranium by the
ton and carefully isolated the radioactive components
over several years. What they found was that there were more
than one radioactive element in it. Because of this she
got the nobel prize in physics for radioactivity and a nobel
prize in chemistry for discovering two new elements.

Their Place of work

J.J Thompson (1856-1940)
Year of Discovery- 1898
Country of Origin-England

J.J Thompson (1856-1940)
Plum pudding model of the atom
By the end of the nineteenth century, scientists were convinced that matter was composed of atoms, the permanent, industructible building blocks from which all substances are constructed. However, an English physicist named J.J. Thompson (1856-1940) complicated the picture by discovering an even smaller and more functional particle called the electron. Thompson discovered that electrons are negatively charged, that they are much smaller and lighter than atoms, and that they are uniformly present in many different kinds of substances. (Introductory Chemistry by Nivaldo J. Tro p.93)

The plum pudding model

The Plum Pudding Model of the atom was proposed in 1904 before the discovery of the atomic nucleus. In this model, the atom is composed of electrons surrounded by a soup of positive charge to balance the electron's negative charge, like negatively-charged "plums" surrounded by positively-charged "pudding".


Ernest Rutherford (1871-1937)
Year of Discovery- 1909
Country of Origin-England
Ernest Rutherford
In 1909, Ernest Rutherford's experiment proved Thompson
wrong. In his experiment, Rutherford directed tiny,
positively charged particles, alpha particles, at an
ultrathin sheet of gold foil. These particles were to act as
probes of the gold atoms structure. If the gold atoms were
indeed like blueberry muffins, or plum pudding-- with their
mass and charge spread throughout the entire volume of the
atom-- these speeding probes should pass right through the
gold foil with minimun deflection. In Rutherfords experiment
a majority of the particles did pass directly though the foil,
but some particles were deflected, and some (1 in 20,000)
even bounced back. He concluded that matter has large
regions of empty space dotted with small regions of very
dense matter.

He proposed the nuclear theory of the atom.
1.) Most of the atom's mass and all of its positive
Rutherford's Gold Foil Experiment

charge are contained in a small core called the nucleus.
2.) Most of the volume of the atom is empty space
through which the tiny, negatively charged electrons are dispersed.
3.) There are as many negatively charged electrons outside
the nucleus as there are positively charged particles (protons)
inside the nucleus, so that the atom is electrically neutral.

Albert Einstein 1879-1955
Year of Discovery- 1905 zz_bert.jpg
Country of Origin-Germany

Albert Einstein
Einstein is one of the greatest scientific minds of all time.
He is best known for his theory of relativity and specifically
mass–energy equivalence, E = mc², the most famous
equation of the twentieth century. Einstein received the
1921 Nobel Prize in Physics. He was credited with the development of the Theory of Relativity. He found that
waves of particles move in relation of their properties.
He also found that radioactive elements contained bounds of energy.When an atom is split in a radioactive element, the
energy released is exponential. He was credited
with the development of the Atom Bomb.

Robert Millikan (1868-1953)
Year of Discovery- 1908
Country of Origin-USA
Millikan's "cloud chamber"

Robert Millikan
Millikan made numerous momentous discoveries in the fields of electricity, optics, and molecular physics. His earliest major success was the accurate determination of the charge carried by an electron, using the elegant "falling-drop method" he also proved that this quantity was a constant for all electrons, demonstrating the atomic structure of electricity. Millikan occupied himself with work concerning the hot-spark spectroscopy of the elements (which explored the region of the spectrum between the ultraviolet and X-radiation), thereby extending the ultraviolet spectrum downwards far beyond the then known limit. The discovery of his law of motion with his other investigations on electrical phenomena, led him to his significant studies of cosmic radiation.
Cross section of the chamber

Between 1909 and 1916 Millikan devised increasingly precise ways to demonstrate that there is a smallest possible electric charge and to measure that charge. Earlier Charles Wilson in England had devised what became known as a cloud chamber to visualize the path of charged particles by a trail of droplets. After an observation, the trail of charged droplets could be cleared from the apparatus by applying an electric field arranged by parallel plates. But in air, the tiny droplets quickly reach a terminal velocity, and then travel at constant velocity. The accelerating force on their electric charge quickly is balanced by friction determined by the droplet size. Millikan realized that the experiment could be turned around so by measuring the size and terminal speed, he could calculate the electric charge on each droplet. But condensation or evaporation often changed the tiny droplet size complicating the calculations. While on a train journey to a convention, Millikan glanced at his watch and realized that the clock oil used for long lasting lubrication was chosen for its lack of evaporation! Substituting a fine mist of oil droplets, Millikan demonstrated that all measured electric charges were multiples of 1.6 10-19 Coulombs. A droplet could have an excess or deficiency of one or more electrons, but never gain or lose a fraction of an electron. This led to the mass of the electron being determined to be 9.1 x 10-31 kg.

Planetary Model- 1913

zz_plant.jpgIntroduced by Niels Bohr in 1913, the model's key success lay in explaining the Rydberg formula for the spectral emission lines of atomic hydrogen; while the Rydberg formula had been known experimentally, it did not gain a theoretical underpinning until the Bohr model was introduced. Not only did the Bohr model explain the reason for the structure of the Rydberg formula, but it provided a justification for its empirical results in terms of fundamental physical constants.

The Rydberg formula is used in atomic physics to describe the wavelengths of spectral lines of many chemical elements. The formula was invented by the Swedish physicist Johannes Rydberg.
frac{1}{lambda_{mathrm{vac}}} = R_{mathrm{H}} left(frac{1}{n_1^2}-frac{1}{n_2^2}right)
frac{1}{lambda_{mathrm{vac}}} = R_{mathrm{H}} left(frac{1}{n_1^2}-frac{1}{n_2^2}right)

Rutherford-Bohr Model-1915

rutherford-bohr model
A New Zealand scientist, Ernest Rutherford, and a Danish scientist, Niels Bohr, developed a way of thinking about the structure of an atom in which an atom looks very much like our solar system.; It is known as the Rutherford-Bohr Theory of Atomic Structure, and was something of a breakthrough
in describing the way the atom works.Rutherford had
conducted experiments in which he shot relatively large,
sun solar system
sun solar system

charged particles (alpha particles) at a thin gold foil. He
found that most of the particles passed directly through the
foil, but some came off at odd angles, as though they had
been deflected. From these results, Rutherford concluded
that each atom was mostly empty space, but also contained
a dense region--a central mass, which his alpha particles
could not pass through. He also concluded that this central
mass must have a positive charge, to deflect the positively
charged alpha particles. Rutherford and Bohr pictured the arrangement of the atom's parts to look like
our solar system. At the center of every atom is a nucleus, which is comparable to the sun in our solar
system. Electrons move around the nucleus in "orbits" similar to the way planets move around the sun.

Niels Bohr (1885-1962)
Year of Discovery- 1913
Country of Origin-Denmark

Neils Bohr
Danish physicist who proposed a successful quantum model of the atom in
The Bohr Atom

1913. His model assumed that
(1) the electron exists at precise distances from the nucleus,
(2) as long as an electron remains in one location, no energy is given off
(3) electrons have circular orbits
(4) the angular momenta associated with allowed electron motion are integral multiples of external image bimg8.gif.
Bohr stated the Correspondence
Principle, which states that quantum mechanical formulas must reduce to
the classical results in the limit of large quantum number.

Bohr on the Simpsons

Niels Henrik David Bohr
Born: 7 October 1885
Died: 18 November 1962
Country of Origin: Denmark
Year of Discovery: 1913
He was a Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. Bohr mentored and collaborated with many of the top physicists of the century at his institute in Copenhagen. He was also part of the team of physicists working on the Manhattan Project. Bohr married Margrethe Nørlund in 1912, and one of their sons, Aage Niels Bohr, grew up to be an important physicist who, like his father, received the Nobel prize, in 1975. Bohr has been described as one of the most influential physicists of the 20th century.

Electron Cloud Model

Electron Cloud Model

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., Each state has a characteristic shape and energy, all predicted by the Schrödinger equation, which has infinitely many solutions. The models in the photograph above represent the real parts of the angular portions of sixteen wavefunctions for an electron in a one-electron atom. The angular portions of hydrogenlike wavefunctions, which are the eigenfunctions of the Schrödinger equation for an electron bound to a nucleus. Niels Bohr developed the first successful quantum theory of the hydrogen atom. He based this theory on the classical model of the electron traveling in circular orbits about the nucleus, but with the electron constrained only to certain allowed orbits. Arnold Sommerfeld then expanded Bohr’s model to yield sets of elliptical orbits. The squares of the functions depicted by the wooden models shown above would yield probability contours, that is, surfaces along which there is a particular (fixed) probability of finding the electron, and which enclose regions within which the total probability of finding the electron is a particular value, so it is not really proper to speak of the motion of the electron as an orbit.

Erwin SchrOdinger (1877-1961)
Year of Discovery- 1926
Country of Origin- Austria

Erwin SchrOdinger

Schrödinger began to think about explaining the movement of an electron in an atom as a wave. By 1926 he published his work, providing a theoretical basis for the atomic model that Niels Bohr had proposed based on laboratory evidence. The equation at the heart of his publication became known as Schrödinger's wave equation. This was the second theoretical explanation of electrons in an atom, following Werner Heisenberg's matrix mechanics. Many scientists preferred Schrödinger's theory since it could be visualized, while Heisenberg's was strictly mathematical. A split threatened among physicists, but Schrödinger soon showed that the two theories were identical, only expessed differently.

movement of an electron in an atom as a wave

Werner Heisenberg (1901-1976)
Year of Discovery- 1939
Country of Origin-Germany

Werner Heisenberg

matrix mechanics
matrix mechanics

Werner was a German theoretical physicist, best known for enunciating the uncertainty principle of quantum theory. He made important contributions to quantum mechanics, nuclear physics, quantum field theory, and particle physics.

James Chadwick (1891-1974)
Year of Discovery- 1932
Country of Origin-England

James Chadwick
Rutherford succeeded in disintegrating atoms by bombarding nitrogen with alpha particles, with the emission of a proton. This was the first artificial nuclear transformation. In Cambridge, Chadwick joined Rutherford in accomplishing the transmutation of other light elements by bombardment with alpha particles, and in making studies of the properties and structure of atomic nuclei.
In 1932, Chadwick made a fundamental discovery in the domain
of nuclear science: he proved the existence of neutrons - zznbhr.jpg
elementary particles devoid of any electrical charge. In contrast
with the helium nuclei (alpha rays) which are charged, and
therefore repelled by the considerable electrical forces present
in the nuclei of heavy atoms, this new tool in atomic disintegration
need not overcome any electric barrier and is capable of
penetrating and splitting the nuclei of even the heaviest elements.
Chadwick in this way prepared the way towards the fission of
uranium 235 and towards the creation of the atomic bomb. For
this epoch-making discovery he was awarded the Hughes Medal
of the Royal Society in 1932, and subsequently the Nobel Prize
for Physics in 1935.

Chadwick's Experiment

Paul Dirac (1902-1984)
Year of discovery-1928
Country of Origin- Britain

Paul Dirac
Ex. Quatum Mechanics
Ex. Quatum Mechanics

Paul Dirac is the founder of the field of quantum mechanics.
Dirac noticed an analogy between the Poisson brackets of
classical mechanics and the recently proposed quantization
rules in Werner Heisenberg's matrix formulation of quantum
mechanics. This observation allowed Dirac to obtain the
quantization rules in a novel and more illuminating manner.
For this work, published in 1926, he received a Ph.D. from

Aage Niels Bohr
Year of Discovery- 1975
Country of Origin- Denmark
his book
his book

Aage Bohr
Aage Bohr

Like father like son. The Bohrs were great physicists who made excellent contibutions.
A major part of our efforts has been connected with the attempt to present the status
of our understanding of nuclear structure in a monograph, of which Volume I
(Single-Particle Motion) appeared in 1969, and Volume II (Nuclear Deformations)
in 1975. For which he recived the nobel prize.


Introductory Chemistry, by Nivaldo J. Tro - Third Edition

PHOTOS jdsymbols.jpg