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A Timeline on Atomic Structure 400 B.C. Democritus’ atomic theory posited that all matter is made up small indestructible units he called atoms. 1704 Isaac Newton theorized a mechanical universe with small, solid masses in motion. 1803 John Dalton proposed that elements consisted of atoms that were identical and had the same mass and that compounds were atoms from different elements combined together. 1832 Michael Faraday developed the two laws of electrochemistry. 1859 J. Plucker built one of the first cathode-ray tubes. 1869 Dmitri Mendeleev created the periodic table. 1873 James Clerk Maxwell proposed the theory of electromagnetism and made the connection between light and electromagnetic waves. 1874 G.J. Stoney theorized that electricity was comprised of negative particles he called electrons. 1879 Sir William Crookes’ experiments with cathode-ray tubes led him to confirm the work of earlier scientists by definitively demonstrating that cathode-rays have a negative charge. 1886 E. Goldstein discovered canal rays, which have a positive charge equal to an electron. 1895 Wilhelm Roentgen discovered x-rays. 1896 Henri Becquerel discovered radiation by studying the effects of x- rays on photographic film. 1897 J.J. Thomson determined the charge to mass ratio of electrons. 1898 Rutherford discovered alpha, beta, and gamma rays in radiation. 1898 Marie Sklodowska Curie discovered radium and polonium and coined the term radioactivity after studying the decay process of uranium and thorium. 1900 Max Planck proposed the idea of quantization to explain how a hot, glowing object emitted light.

A Timeline on Atomic Structure

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A Timeline on Atomic Structure400 B.C.Democritus atomic theory posited that all matter is made up small indestructible units he called atoms.1704Isaac Newtontheorized a mechanical universe with small, solid masses in motion.1803John Daltonproposed that elements consisted of atoms that were identical and had the same mass and that compounds were atoms from different elements combined together.1832Michael Faradaydeveloped the two laws of electrochemistry.1859J. Pluckerbuilt one of the first cathode-ray tubes.1869Dmitri Mendeleevcreated the periodic table.1873James Clerk Maxwellproposed the theory of electromagnetism and made the connection between light and electromagnetic waves.1874G.J. Stoneytheorized that electricity was comprised of negative particles he called electrons.1879Sir William Crookes experiments with cathode-ray tubes led him to confirm the work of earlier scientists by definitively demonstrating that cathode-rays have a negative charge.1886E. Goldsteindiscovered canal rays, which have a positive charge equal to an electron.1895Wilhelm Roentgendiscovered x-rays.1896Henri Becquereldiscovered radiation by studying the effects of x-rays on photographic film.1897J.J. Thomsondetermined the charge to mass ratio of electrons.1898Rutherforddiscovered alpha, beta, and gamma rays in radiation.1898Marie Sklodowska Curiediscovered radium and polonium and coined the term radioactivity after studying the decay process of uranium and thorium.1900Max Planckproposed the idea of quantization to explain how a hot, glowing object emitted light.1900Frederick Soddycame up with the term "isotope" to explain the unintentional breakdown of radioactive elements.1903Hantaro Nagaokaproposed an atomic model called the Saturnian Model to describe the structure of an atom.1904Richard Abeggfound that inert gases have a stable electron configuration.1906Hans Geigerinvented a device that could detect alpha particles.1914H.G.J. Moseleydiscovered that the number of protons in an element determines its atomic number.1919Francis William Astonused a mass spectrograph to identify 212 isotopes.1922Niels Bohrproposed an atomic structure theory that stated the outer orbit of an atom could hold more electrons than the inner orbit.1923Louis de Broglieproposed that electrons have a wave/particle duality.1929Cockcroft / Waltoncreated the first nuclear reaction, producing alpha particles1930Paul Diracproposed the existence of anti-particles.1932James Chadwickdiscovered neutrons, particles whose mass was close to that of a proton.1938Lise Meitner, Hahn, Strassmandiscovered nuclear fission.1941-51Glenn Seaborgdiscovered eight transuranium elements.1942Enrico Fermicreated the first man-made nuclear reactor.

Early theoriesof the structure of matter were not based upon experiments. As scientists began to study the relationship between several physical phenomenon such as electricity, and magnetism they began to develop different models about atomic structure.

Year Scientist(s) Discovery

GreekeraDemocritus"by convention bitter, by convention sweet, but in reality atoms and void"

1704Isaac NewtonProposed a mechanical universe with small solid masses in motion.

1803John DaltonProposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass.

1832Michael FaradayStudied the effect of electricity on solutions, coined term "electrolysis" as a splitting of molecules with electricity, developed laws of electrolysis. Faraday himself was not a proponent of atomism.

1859J. PluckerBuilt one of the first gas discharge tubes ("cathode ray tube").

1869Dmitri MendeleevArranged elements into 7 groups with similar properties. He discovered that the properties of elements "were periodic functions of the their atomic weights". This became known as the Periodic Law.

1873James Clerk MaxwellProposed electric and magnetic fields filled the void.

1879Sir William CrookesDiscovered cathode rays had the following properties: travel in straight lines from the cathode; cause glass to fluoresce; impart a negative charge to objects they strike; are deflected by electric fields and magnets to suggest a negative charge; cause pinwheels in their path to spin indicating they have mass.

1886E. GoldsteinUsed a CRT to study "canal rays" which had electrical and magnetic properties opposite of an electron.

1894G.J. StoneyProposed that electricity was made of discrete negative particles he calledelectrons". (Link to info on electrons)

1895Wilhelm RoentgenUsing a CRT he observed that nearby chemicals glowed. Further experiments found very penetrating rays coming from the CRT that were not deflected by a magnetic field. He named them "X-rays".

1896Henri BecquerelWhile studying the effect of x-rays on photographic film, he discovered some chemicals spontaneously decompose and give off very pentrating rays.

1897J.J. ThomsonUsed aCRTto experimentally determine the charge to mass ratio(e/m)of an electron =1.759 x 10 8 coulombs/gram.

1897J.J. ThomsonStudied "canal rays" and found they were associated with the proton H + .

1898RutherfordStudied radiations emitted from uranium and thorium and named themalphaandbeta.

1898Marie Sklodowska CurieStudied uranium and thorium and called their spontaneous decay process "radioactivity". She and her husband Pierre also discovered the radioactive elements polonium and radium.

1900SoddyObserved spontaneous disintegration of radioactive elements into variants he called "isotopes" or totally new elements, discovered "half-life", made initial calculations on energy released during decay.

1900Max Planckused the idea of quanta (discrete units of energy) to explain hot glowing matter.

1903NagaokaPostulated a "Saturnian" model of the atom with flat rings of electrons revolving around a positively charged particle.

1904AbeggDiscovered that inert gases had a stable electron configuration which lead to their chemical inactivity.

1905Albert EinsteinPublished the famous equationE=mc 2

1906Hans GeigerDeveloped an electrical device to "click" when hit with alpha particles.

1909R.A. MillikanOil drop experiment determined the charge (e=1.602 x 10 -19 coulomb) and the mass (m = 9.11 x 10 -28 gram) of an electron.

1911Ernest RutherfordUsing alpha particles as atomic bullets, probed the atoms in a piece of thin (0.00006 cm)gold foil. He established that the nucleus was: very dense,very small and positively charged. He also assumed that the electrons were located outside the nucleus.

1914H.G.J. MoseleyUsing x-ray tubes, determined the charges on the nuclei of most atoms. He wrote"The atomic number of an element is equal to the number of protons in the nucleus". This work was used to reorganize the periodic table based upon atomic number instead of atomic mass.

1919AstonDiscovered the existence of isotopes through the use of a mass spectrograph.

1922Niels BohrDeveloped an explanation of atomic structure that underlies regularities of the periodic table of elements. His atomic model had atoms built up of sucessive orbital shells of electrons.

1923de BroglieDiscovered that electrons had a dual nature-similar to both particles and waves. Particle/wave duality. Supported Einstein.

1927HeisenbergDescribed atoms by means of formula connected to the frequencies of spectral lines. Proposed Principle of Indeterminancy - you can not know both the position and velocity of a particle.

1929Cockcroft / WaltonBuilt an early linear accelerator and bombarded lithium with protons to producealpha particles

1930SchrodingerViewed electrons as continuous clouds and introduced "wave mechanics" as a mathematical model of the atom.

1930Paul DiracProposedanti-particles. Anderson discovered the anti-electron (positron) in 1932 and Segre/Chamberlain detected the anti-proton in 1955..

1932James ChadwickUsing alpha particles discovered a neutral atomic particle with a mass close to a proton. Thus was discovered the neutron.

1938Lise Meitner,Hahn, StrassmanConducted experiments verifying that heavy elements capture neutrons and form unstable products which undergo fission. This process ejects more neutrons continuing the fission chain reaction.

1941 - 51Glenn SeaborgSynthesized 6 transuranium elements and suggested a change in the layout of the periodic table.

1942Enrico FermiConducted the first controlled chain reaction releasing energy from the atoms nucleus.

Development of the Atomic Theoryhttp://cstl-csm.semo.edu/cwmcgowan/ch181/atomhist.htm Democritus (460-370 BC) First proposed the existence of an ultimate particle. Used the word "atomos" to describe this particle.

Democritus Aristotle (384-322 BC) was a proponent of the continuum. He believed in the four elements of air, earth, water and fire. Aristotle felt that regardlessofthe number oftimesyou cut a form of matter in half, you would always have a smaller piece of that matter. This view held sway for 2000 years primarily because Aristotle was the tutor of Alexander the Great. JohannBecher (1635-1682) and Georg Stahl (1660-1734) developed the Phlogiston theory which dominated chemistry between 1670 and 1790. Basically, when something burned, it lost phlogiston to the air (after all, you could see the phlogiston leaving) A problem with the theory was that burning of metals resulted in an increase in the mass. This problem was solved by assigning negative mass to phlogiston. Joseph Priestly (1733-1804) discovered oxygen (which he called "dephlogisticated air") in 1774. Priestly was an ardent phlogistonist until his dying day. Priestly was also an early anti-war activist who favored both the American and French Revolutions. He was shipped to the U.S. in 1791 where he lived a quiet life in Pennsylvania. His house was used as a starting point for the American Chemical Society in 1876. The Priestly Medal is the highest award given by to an American chemist by the Society.Joseph Priestly Antoine Lavoisier (1743-1794) was the first person to make good use of the balance. He was an excellent experimenter. After a visit with Priestly in 1774, he began careful study of the burning process. He proposed the Combustion Theory which was based on sound mass measurements. He named oxygen. He also proposed the Law of Conversation of Mass which represents the beginning ofmodern chemistry. To support his work, Lavoisier was associated with a tax-collecting firm and was married to the daughter of the one of the firm's executives. Some people believe that Madame Lavoisier was every bit as good a scientist as her husband. Unfortunately, this relationship with the tax firm led to Lavoisier's beheading at the guillotine in 1794.Marie and Antoine Lavoisier Joseph Proust (1754-1826) proposed the the Law of Constant Composition in 1799. This law was very radical at the time and was hotly contested by Claude Berthollet (1748-1822).Joseph Proust John Dalton (1776-1844) proposed the Law of Multiple Proportions. This law led directly to the proposal of the Atomic Theory in 1803. He also developed the concept of the mole and proposed a system of symbols to represent atoms of different elements. (The symbols currently used were developed by J.J. Berzelius(1779-1848)). Dalton recognized the existence of atoms of elements and that compounds formed from the union of these atoms. He therefore assumed that simplest ratios would be used in nature and came up with a formula for water of HO. He then assigned a relative atomic weight of one to hydrogen and developed a relative atomic weight scale from percent composition data and assumed atomic ratios. Today we would refer to these as equivalent masses. John Dalton also discovered color blindness, an affliction from which he suffered. He determined that five percent of the male population and less than one-tenth percent of the female population was color blind. Joseph Gay-Lussac ( 1778-1850) announced the Law of Combining Volumes in 1808. He showed that at the same temperature and pressure, two volumes of hydrogen gas reacted with one volume of oxygen gas to produce two volumes of water (as a gas).Joseph Gay-Lussac Amadeo Avogadro (1776-1856) proposed what is now known as Avogadro's Hypothesis in 1811. The hypothesis states that at the same temperature and pressure, equal volumes of gases contain the same number of molecules or atoms. When this is combined with Gay-Lussac's Law of Combining Volumes, the only possible formulas for hydrogen, oxygen and water are H2, O2and H2O, respectively. The solution to the atomic weight problem was at hand in 1811. However, Avogadro's Hypothesis was a radical statement at the time and was not widely accepted until fifty years later. Stanislao Cannizzaro (1826-1910), in 1860 at the Karlsruhe Conference, proposed that Avogadro's Hypothesis be accepted and the implications used for a period of five years. At the end of this five year period, a new conference would be called to discuss any problems that might develop; this second conference was never called.

Stanislao Cannizzaro

Dimitri Mendeleev (1834-1907) proposed the periodic law and developed the first periodic table in 1869. Medeleev's table was arranged according to increasing atomic weight and left holes for elements that were yet to be discovered.

Dimitri MendeleevDevelopment of Atomic Structure J. J. Thomson (1856-1940) identified the negatively charged electron in the cathode ray tube in 1897. He deduced that the electron was a component of all matter and calculated the charge to mass ratio for the electron. e/m = -1.76 x 108coulombs/gThomson and others also studied the positive rays in the cathode ray tube and discovered that the charge to mass ratio depended on filling gas in the tube. The largest charge to mass ratio (smallest mass) occurred when hydrogen was the filling gas. This particle was later identified as the proton. e/m = +9.58 x 104coulombs/gThomson proposed the "plum pudding" model of the atom. In this model, the volume of the atom is composed primarily of the more massive (thus larger) positive portion (the plum pudding). The smaller electrons (actually, raisins in the plum pudding ) are dispersed throughout the positive mass to maintain charge neutrality.

Joseph John Thomson Robert Millikan (1868-1953) determined the unit charge of the electron in 1909 with his oil drop experimentat the University of Chicago. Thus allowing for the calculation of the mass of the electron and the positively charged atoms. e = 1.60 x 10-19coulombs

Robert Millikan Ernst Rutherford (1871-1937) proposed the nuclear atom as the result of the gold-foil experiment in 1911. Rutherford proposed that all of the positive charge and all of the mass of the atom occupied a small volume at the center of the atom and that most of the volume of the atom was empty space occupied by the electrons. This was a very radical proposal that flew in the face of Newtonian Physics. Although positive particles had been discussed for some time, it was Rutherford in 1920 that first referred to the hydrogen nucleus as a proton. Also in 1920, Rutherford proposed the existence of the third atomic particle, the neutron. Henry Moseley (1887-1915) discovered that the energy of x-rays emitted by the elements increased in a linear fashion with each successive element in the periodic table. In 1913, he proposed that the relationship was a function of the positive charge on the nucleus. This rearranged the periodic table by using the atomic number instead of atomic mass to represent the progression of the elements. This new table left additional holes for elements that would soon be discovered. Unfortunately, Moseley was killed at Gallipoli during WWI.

Francis Aston (1877-1945) invented the mass spectrograph in 1920. He was the first person to observe isotopes. For example he observed that there were three different kinds of hydrogen atoms. While most of the atoms had a mass number of 1 he also observed hydrogen atoms with mass numbers of 2 and 3. Modern atomic masses are based on mass spectral analysis. His work led Rutherford to predict the existence of the neutron.

Francis Aston James Chadwick (1891-1974) discovered the neutron in 1932. Chadwick was a collaborator of Rutherford's. Interestingly, the discovery of the neutron led directly to the discovery of fission and ultimately to the atomic bomb.

James Chadwick