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Elizabeth Weis Haley Tierce Chemistry-6 th


 Formulated the atomic theory; the first to describe invisible  

"atoms" as the basis of all matter. 370 – 460 BCE The theory of Democritus and his mentor, Leucippus, held that everything is composed of "atoms", which are physically; that between atoms lies empty space; that atoms are indestructible; have always been, and always will be, in motion; that there are an infinite number of atoms, and kinds of atoms, which differ in shape, and size. Leucippus is sometimes credited with being the first to discover it but there have been many more to claim inventing it. In the end they all kept building ideas up and up until modern science was able to verify their conclusions. Many consider Democritus to be the "father of modern science".


 Pioneered the development of the modern atomic theory  1803  Dalton expanded the ideas from Democritus and proposed

these ideas: (this was his Atomic Theory)

 The atoms of a given element are different from those of any

other element; the atoms of different elements can be distinguished from one another by their respective relative atomic weights.  All atoms of a given element are identical.  Atoms cannot be created, divided into smaller particles, nor destroyed in the chemical process; a chemical reaction simply changes the way atoms are grouped together.  Elements are made of tiny particles called atoms. He also came up with the idea that when elements are chemical combined it takes place between particles of different weights.


 His 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, most of Dalton’s principles are still used today.  He was color blind


 Spectrometer  1859  With Gustav Robert Kirchhoff, he observed that

each element emits light of a characteristic wavelength, opening the field of spectro-chemical analysis.  The spectrometer led to his discovery of the elements cesium and rubidium  Although often accredited to have invented the Bunsen burner, he only made a contribution to it


 Created the first version of the periodic table  1869  As he attempted to classify the elements according to their

chemical properties, he noticed patterns that led him to hypothesize his Periodic Table. He discovered the elements, if arranged according to their atomic weight, exhibit an apparent periodicity of properties and elements that have similar atomic weights have similar chemical properties.  Mendeleev’s guesses were sometimes off because of the way atomic weights were calculated. Mendeleev formed his hypothesis on the simple one-to-one proton/neutron-ratio version of the element not the weighted average of all of an element's common isotopes.  The crater Mendeleev on the Moon, as well as element number 101, the radioactive mendelevium, are named after him.


 Discovered anode rays  1886  In 1886, Goldstein discovered that discharge tubes emit a glow at

the cathode end. He concluded that in addition to the alreadyknown cathode rays, there is another ray that travels in the opposite direction. Because these last rays passed through the holes, or channels, in the cathode, Goldstein called them kanalstrahlen, or canal rays. They are composed of positive ions whose identity depends on the residual gas inside the tube.  The cathode rays were later recognized as electrons moving from the negatively-charged cathode toward the positively-charged anode.  Goldstein's atomic theory was similar to the modern one, but because of conflicting opinions of his colleagues, he was (and is) commonly disregarded.


 Electron  1891  Stoney suggested a name for the fundamental unit of

electricity, whether particle or not. He suggested the name electron.  In 1891, he proposed the term 'electron' to describe the fundamental unit of electrical charge, and his contributions to research in this area laid the foundations for the eventual discovery of the particle by J.J. Thomson in 1897.  Stoney was the uncle of George FitzGerald, who was also a brilliant mathematical physicist


 Discovered the electron and isotopes and invented the mass  

spectrometer. 1897 Thomson discovered the electron through his explorations on the properties of cathode rays. Thomson found that the rays could be deflected by an electric field. By comparing the deflection of a beam of cathode rays by electric and magnetic fields he was able to measure the particle's mass. This showed that the particles were about 2000 times lighter than the mass of the lightest atom, hydrogen. He concluded that the rays were composed of very light negatively charged particles which he called "corpuscles“ later renamed the electron. Thomson imagined the atom as being made up of these corpuscles swarming in a sea of positive charge. This model was later proved incorrect when Ernest Rutherford showed that the positive charge is concentrated in the nucleus of the atom. Thompson was admitted to Owens College when he was 14 years old because of his great interest and ability in science.


 Valence Theory  1902-1904  From his study of valence, he found that some elements were less

likely to combine into molecules, and from this concluded that the more stable elements had what are now called full electron shells. He was able to explain the attraction of atoms through opposite electrical charges. He also made the distinction between normal valence and contravalence. He found that the sum of these two valences always comes to eight, a rule that is now known as Abegg's rule  His theory had not been improved  He also enjoyed photography and balloon trips


 Radioactivity  1903  At the Municipal School Marie Curie met Henri Becquerel (1852–1908), a

scientist who studied X rays. Becquerel had observed rays given off by the element uranium. Intrigued, Marie wanted to find out more about these mysterious rays and about uranium. Within a couple of months, she discovered that the intensity of the rays was in direct proportion to the amount of uranium in her sample, and that, regardless of other factors—combining it with other elements, or subjecting it to heat, cold, or light—nothing seemed to affect the rays. She experimented with other elements, and found that another substance, thorium, was also "radioactive"—a term she coined—giving off similar rays to those found in uranium. Together, Marie and Pierre Curie demonstrated that the radioactivity was not the result of a chemical reaction but was rather a property of the atoms that made up the elements.  Her discovery helped create X-rays .  She graduated from high school at 15 .


 published his special theory of relativity (provides an

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understanding of the relationship of matter and energy) 1905 Einstein used math to describe the physics in the universe. While he was working with some mathematical relationships between various equations, he found they would condense into his famous equation: e=mc^2. Albert Einstein’s theory of relativity created the beginnings of the atomic bomb. Einstein suggested that the immense power in the atom’s nucleus could be used to create an atomic bomb. He sent a letter to Franklin D. Roosevelt recommending investigation on forming an atomic bomb. Einstein's work changed contemporary thinking and formed the basis for the modern science of physics.


 Radioactive Isotopes  1908  Frederick Soddy used a method of examining the nature of

gases by bombarding them in an evacuated glass tube with an electric current. These are also known as "Crookes tubes”.  This discovery was not only useful to chemistry but for many other disciplines. The best known use of the isotope is in nuclear weapons and energy which became reality in the 1940s.  He went to Oxford University.


 Quanta  1908  He made experimental observations on the wavelength distribution of

the energy emitted by a black body as a function of temperature were at variance with the predictions of classical physics. Planck was able to deduce the relationship between the energy and the frequency of radiation. In a paper published in 1900, he announced his derivation of the relationship: this was based on the revolutionary idea that the energy emitted by a resonator could only take on discrete values or quanta.  Albert Einstein used Planck concept to explain photoelectricity in 1905, and Niels Bohr applied Planck concept to the atom itself in 1913.  He had a doctorate of philosophy .


 Measured the charge of the electron  1909  Preformed the oil-drop experiment where he balanced gravity’s

downward pull and buoyancy's upward pull on tiny charged droplets of oil suspended between two metal electrodes (electrical conductors used to make contact with a nonmetallic part of a circuit). Since the density of the oil was known, the droplets' masses, and therefore their gravitational and buoyant forces, could be determined. This allowed him to determine the charge of an electron when all the particles of the system are at rest and the total force on each particle is permanently zero while using a know electric field. He confirmed it by repeating the procedure many times.  Millikan’s value for the charge of an electron was (1.592 × 10−19 coulomb). This is somewhat lower than the modern value of 1.602 176 53(14) x 10−19 coulomb and is probably due to Millikan's use of an inaccurate value for the viscosity of air.  Millikan won the Nobel prize in physics.


 X-ray spectra to study atomic structure  1913  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.  Moseley's discoveries resulted in a more accurate positioning of elements in the Periodic Table by closer determination of atomic numbers.  He was awarded a King’s scholarship.


• Atomic Structure • 1913 and 1922 • Bohr discovered that Ernest Rutherford's idea of the structure of an

atom had a few problems. Physics proves that the electrons orbiting the nucleus should be losing energy until they spiral down and collide with the nucleus making all atoms unstable. Bohr acknowledged that electrons existed at set levels of energy that’s at fixed distances from the nucleus. • Niels Bohr's atomic model was too simple and insufficient to describe atoms, but it opened the door of modern atomic physics with the introduction of the quantized energy levels around the atom. It's impressive to note the fast and great progress that atomic physics have made since then. Nevertheless, the main products of these experiments were not positive for mankind. They produced atomic bombs and nuclear fission to produce energy, with the unresolved problem of radioactive nuclear wastes and leaks from the reactors. • He won the Nobel Prize in 1922.


 Atomic Structure  1918  Ernest Rutherford's experiment was fairly simple, he

took an alpha emitting source and fired the alpha particles at a layer of gold foil which was only a few atoms across and by placing a detector at the other side of the foil he discovered that most of them passed straight through, but not all of them.  This model has been expanded on since this was created.  Rutherford was knighted in 1914.


 Stated that any moving particle or object had an

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associated wave; thus he created new field in physics: wave mechanics. 1924 He united the physics of energy (wave) and matter (particle). de Broglie worked to develop an explanation of wave mechanics, opposing the entirely problematic models which dominate quantum mechanical theory. In the 1950s David Bohm refined his theory. He won the Nobel Prize in Physics in 1929.


 Pioneered quantum physics and discovered a new law of  

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nature 1924 He clarified the existing knowledge of atomic structure at the time and it led to the recognition of the two-valued variable required to characterize the state of an electron. Pauli was the first to recognize the existence of the neutrino, an uncharged and mass less particle which carries off energy in radioactive disintegration. There have been many advances in quantum physics that were made because of Pauli. In 1945, Wolfgang Pauli was nominated for Nobel Prize in Physics by Albert Einstein.


 made contributions to quantum theory about the  

structure of the atom; developed “Hund’s rules”. 1925 Hund's rules can be understood by assuming that electrons try to stay as far apart as possible to minimize the force of repulsion between these particles. Hund’s rules can not be applied to all elements. Robert Millikan was greatly influenced by Hund .


• Electron cloud model of the atom • 1926 • Schrödinger's wave equation was based on the Heisenberg

uncertainty principal that the position and velocity of a electron cannot be determined accurately (accuracy in one will sacrifice accuracy in another). Schrödinger's wave equation gave a wave function, which squared gave the probability cloud of electrons. Therefore, Schrödinger's contribution resulted in the electron cloud model of the atom. • Nothing has improved this model. • He won the Nobel Prize for Physics in 1933.


 Principle of Uncertainty  1927  This principle states that it is impossible to determine

both the position and the momentum of the particle at the same time.  This principle is still used and had not been replaced it helps with quantum physics.  His father was a doctor .


 Discovered the neutron  1932  Prior to 1932, the accepted model of the atom contained only positively

charged protons and negatively charged electrons. Physicist Sir James Chadwick noticed a inconsistency between the atomic numbers of atoms and the atomic weight. Bombarding base elements with other high energy particles would cause them to break apart. When they break apart, their particles would collide with other particles. By measuring the velocity of the struck particles, Chadwick was able to determine the mass of the radiation and was actually extremely close to that of a proton. The newly discovered neutron had a mass within 1% of the proton. Thus he proved that there was a neutral particle in the nucleus that took up space and had weight but didn’t have a charge.  There have been greater advances in discovering the exact characteristics of the neutron.  Chadwick was knighted in 1945.


Atomic Theory