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Atomic Theory Project By Reece Willoughby, Jovan Lopez, Eric Martinez

James Chadwick - Discoverer of the Neutron (1922)


James Chadwick used the apparatus depicted above to discover the neutron. The polonium source emits alpha (α) particles. The particles strike a sample of beryllium, resulting in the emission of a neutron (n0). The ejected neutrons hit the target material—paraffin, for instance—and eject a proton that is recorded by the detection device. There are no disproval’s of his theory though. Interesting Fact: Chadwick participated in the efforts to devise the atomic bombs

Eugen Goldstein – Sugested the idea of Protons and Founder of Properties of Cathode Rays. (18501930) German physicist Eugen Goldstein studied under Hermann von Helmholtz, and coined the term "cathode rays", for the negatively-charged electrons discovered by Johann Hittorf, emitted when electric current is forced through a vacuum tube. He discovered anode rays (also called canal rays), the positively-charged particles formed when electrons are removed from the gas particles in a cathode-ray tube. He showed that cathode rays can cast shadows, then demonstrated how such rays are emitted, perpendicular to the cathode surface, and can be manipulated with magnetic fields. Goldstein's work suggested the presence of the proton -- a positively charged particle, later discovered by Ernest Rutherford. Interesting Fact: Goldstein's atomic theory was very similar to the modern one, but because of differing opinions of his colleagues, he was (and is) widely ignored.

Democritus – Discovered Atomic Theory of the Cosmos (460B.C.E.-370B.C.E.)


The Democritean atom is an inert solid (merely excluding other bodies from its volume) that interacts with other atoms mechanically. In contrast, modern, quantum-mechanical atoms interact via electric and magnetic force fields and are far from inert. His exact contributions are difficult to disentangle from his mentor Leucippus, as they are often mentioned together in texts. Interesting Fact: Democritus was a Greek philosopher as well as a mathematician and scientist.

Robert Millikan – Discoverer of the Electron (1909) The experiment is called the "oil-drop experiment" and it was the first successful scientific attempt to detect and measure the effect of an individual subatomic particle. The scheme of the experiment is as follows: An atomizer sprayed a fine mist of oil droplets into the upper chamber. Some of these tiny droplets fell through a hole in the upper floor into the lower chamber of the apparatus. Millikan first let them fall until they reached terminal velocity due to air resistance. Using the microscope, he measured their terminal velocity, and by use of a formula, calculated the mass of each oil drop. Next, Millikan applied a charge to the falling drops by irradiating the bottom chamber with x-rays. This caused the air to become ionized, which basically means that the air particles lost electrons. A part of the oil droplets captured one or more of those extra electrons and became negatively charged. By attaching a battery to the plates of the lower chamber he created an electric field between the plates that would act on the charged oil drops; he adjusted the voltage till the electric field force would just balance the force of gravity on a drop, and the drop would hang suspended in mid-air. Some drops have captured more electrons than others, so they will require a higher electrical field to stop. Particles that did not capture any of the extra electrons were not affected by the electrical field and fell to the bottom plate due to gravity. When a drop is suspended, its weight m · g is exactly equal to the electric force applied, the product of the electric field and the charge q · E. The values of E (the applied electric field), m (the mass of a drop which was already calculated by Millikan), and g (the acceleration due to gravity), are all known values. So it is very easy to obtain the value of q, the charge on the drop, by using the simple formula: m·g=q·E Millikan repeated the experiment numerous times, each time varying the strength of the x-rays ionizing the air, so that differing numbers of electrons would jump onto the oil molecules each time. He obtained various values for q. The charge q on a drop was always a multiple of 1.59 x 10-19 Coulombs. This is less than 1% lower than the value accepted today: 1.602 x 10-19 C


Interesting Fact: For his experiment and work on the photoelectric effect Robert Millikan won the 1923 Nobel Prize in physics. J.J. Thomson – Discoverer of Electron (1897) He discovered the electron in a series of experiments designed to study the nature of electric discharge in a high-vacuum cathode-ray tube, an area being investigated by numerous scientists at the time. Thomson interpreted the deflection of the rays by electrically charged plates and magnets as evidence of "bodies much smaller than atoms" that he calculated as having a very large value for the charge-to-mass ratio. Later he estimated the value of the charge itself. In 1904 Thomson suggested a model of the atom as a sphere of positive matter in which electrons are positioned by electrostatic forces. His efforts to estimate the number of electrons in an atom from measurements of the scattering of light, X, beta, and gamma rays initiated the research trajectory along which his student Ernest Rutherford moved. Thomson's last important experimental program focused on determining the nature of positively charged particles. Here his techniques led to the development of the mass spectrograph. His assistant, Francis Aston, developed Thomson's instrument further and with the improved version was able to discover isotopes—atoms of the same element with different atomic weights —in a large number of nonradioactive elements Interesting Fact: One of his students was Ernest Rutherford, who would later succeed him in the post. John Dalton – The Law of Simple Multiple Proportions (Late 18th Century to early 19th Century) His atomic theory, stated that elements consisted of tiny particles called atoms. He said that the reason an element is pure is because all atoms of an element were identical and that in particular they had the same mass. He also said that the reason elements differed from one another was that atoms of each element were different from one another; in particular, they had different masses. He also said that compounds consisted of atoms of different elements combined together. Compounds are pure substances (remember they cannot be separated into elements by phase changes) because the atoms of different elements are bonded to one another somehow, perhaps by hooks, and are not easily separated from one another. Compounds have constant composition because they contain a fixed ratio of atoms and each atom has its own characteristic weight, thus fixing the weight ratio of one element to the other. In addition he said that chemical reactions involved the rearrangement of combinations of those atoms.

Interesting Fact: John Dalton (1766–1844) was born into a modest Quaker family in Cumberland, England, and earned his living for most of his life as a teacher and public lecturer, beginning in his village school at the age of 12.


Dmitri Mendeleev – One of the First arrangements of the Periodic Table (1869) His first Periodic Table was compiled on the basis of arranging the elements in ascending order of atomic weight and grouping them by similarity of properties. He predicted the existence and properties of new elements and pointed out accepted atomic weights that were in error. This organization surpassed attempts at classification by Beguyer de Chancourtois and Newlands and was published a year before the work of Lothar Meyer. Mendeleev provided for variance from strict atomic weight order, left space for new elements, and predicted three yet-to-be-discovered elements including eke-silicon and ekeboron. His table did not include any of the Noble Gases, however, which had not yet been discovered. The original table has been modified and corrected several times, notably by Moseley, but it had accommodated the discovery of isotopes, rare gases, etc. Interesting Fact: Dmitri Mendeleev was born in Tobolsk, Siberia

George Johnstone Stoney - physicist who introduced the term electron for the fundamental unit of electricity. George Stoney estimated the number of molecules in a volume of gas under standard conditions. In 1847 he used sound estimate the value of the electronic charge; however, his result was incorrect because of the number of atoms in a gram of hydrogen was incorrect. Stoney also proposed explanations for the escape of hydrogen and helium from the field of Earth’s gravity and the absence of atmospheres on moon.

Erwin Schrodinger – famous for Schrodinger’s Equation In 1926 Schrodinger founded Schrodinger's Equation which was an adaptation of a proposal made by Louis deBrogile in 1924 that particles of matter have a dual nature and sometimes act like waves. With this Schrodinger’ made his own theory. Newton’s Equations based which are based on the acts of probability, while Schrodinger’s equation are only related to the probable occurrence of actual events. Schrodinger was actually a personal friend of Albert Einstein.


Richard Abegg - Known for the valence theory In 1897 in his study of valence, he concluded that, for the most stable configuration, the number of electrons in the outer group of an atom is eight, which is the number found in the inert gases. While there was no downfall to his experiment he was a gas balloon enthusiast, which caused his death at the age of 41 when he crashed in his balloon.

Marie Curie – Known for her researches in the field of radioactivity She found her interest drawn to pitchblende, a mineral with activity, superior to that of pure uranium. This could only be explained by a presence of an unknown ore. Pierre Curie then joined her in the work that she had undertaken to resolve this problem and that led to the discovery of the new elements, polonium and radium in 1896. There were no downfalls, but she is the only woman to have won the Nobel PeacePrize in 2 different categories.

Frederick Soddy – Working with radioactive isotopes He was among the first to conclude in 1913 that certain elements might exist in forms that differ in atomic weight while being indistinguishable and inseparable chemically. These, upon a suggestion by Margaret Todd, he called isotopes .He discovered isotopes are created by the transmutation of elements through radioactive decay. He showed that elements move two atomic numbers lower by alpha decay and one atomic number by beta decay. There were no downfalls to any of his experiments, but he was known for being handsome.


Max Planck - Plank’s radiation law “He knew how the entropy of the radiation had to depend mathematically upon its energy in the high-frequency region if Wien's law held there. He also saw what this dependence had to be in the low-frequency region in order to reproduce the experimental results there. Planck guessed, therefore, that he should try to combine these two expressions in the simplest way possible, and to transform the result into a formula relating the energy of the radiation to its frequency.” His work was done in 1900. Max Planck had perfect pitch and was a very splendid pianist.

Werner Heisenberg - “ Zur Quantenmechanik” (“On Quantum Mechanics”) Heisenberg did his main work in 1925. He is most famous for working with his assistant and finding that quantum mechanics can be written using Algebra by the way of matrices. There were no downfalls to Werner, but He was also known for being handsome.

Henry Moseley - Moseley’s Law In 1913 he was the English physicist who was most renown for experimentally demonstrating that the major properties of an element are determined by the atomic number, not by the atomic


weight, and brought about the knowledge on relationship between atomic number and the charge of the atomic nucleus. Moseley died at the young age of 27.

Robert Bunsen - Bunsen Burner and more In 1859 he observed that each element emits a light of characteristic wavelength. These studies opened the field of spectrum analysis, which became of great importance in the study of the Sun and stars. Found a cure for arsenic poisoning.He lost an eye trying to find a cure for arsenic poisoning he also invented the Bunsen burner. In his early days in Berlin, Einstein postulated that the correct interpretation of the special theory of relativity must also furnish a theory of gravitation and in 1916 he published his paper on the general theory of relativity. During this time he also contributed to the problems of the theory of radiation and statistical mechanics.

George Johnstone Stoney - physicist who introduced the term electron for the fundamental unit of electricity.


From Stoney’s work on molecular physics and kinetic theory of gases, he estimated the number of molecules in a volume of gas under standard conditions. In 1874 George Stoney used sound to as a method of estimating the value of the electronic charge, his result was incorrect because of an erroneous idea of the number of atoms in a gram of hydrogen. Stoney also proposed explanations for the escape of hydrogen and helium from the field of Earth’s gravity and the absence of atmospheres on moon.

Louis Debroglie – wave mechanics In 1924 he wrote a thesis to obtain his doctor's degree. deBroglie’s thesis led him to fully confirm the discovery of electron diffraction by crystals in 1927 by Davisson and Germer; which served as the basis for developing the general theory of wave mechanics. Between 1930 and 1950, deBroglie's work was focused on various extensions of wave mechanics such as, Dirac's electron theory, the new theory of light, the general theory of spin particles, applications of wave mechanics to nuclear physics. Louis deBroglie wrote many over 25 books.

Friedrich Hund - quantum tunneling Studied under Max Born. He discovered the principle of quantum tunneling in a paper published in 1927. He also did significant work on the structures of atoms and molecules, molecular orbital theory known as the Hund-Mulliken theory. Electrons fill degenerate orbitals according to rules first stated by Hund. His rules assume that electrons try to stay as far apart as possible to minimize the force of repulsion between these particles.


Neils Bohr In 1913, he passed on to a study of the structure of atoms using Rutherford's discovery of the atomic nucleus. Bohr succeeded in working out and presenting a picture of atomic structure that, with later improvements, still serves as an explanation of the physical and chemical properties of the elements.

Wolfgang Pauli – Pauli Exclusion Principle In 1925 he founded the Pauli Exclusion Principle, which states that in an atom no two electrons can occupy the same quantum state simultaneously. When he was 20, Pauli wrote a 200-page encyclopaedia article on the theory of relativity. In 1924 he proposed that a fourth quantum number, which may take on the numerical values +1/2 or -1/2, was necessary to specify electron energy states. Although it was later found that the two values represent the two possible directions of spin for fermions (an simple particle with a half-integral spin that obeys the Pauli exclusion principle. Examples: Electrons, protons, and neutrons).


Atomic Theory PJ  

Atomic Theory Project Mrs G.

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