S: Atomic History
FC: Atomic History Timeline | By: Franchesca Uhar
1: Democritus | (Greek Era) | "by convention bitter, by convention sweet, but in reality atoms and void" | He hypothesized that all matter (including space and time) is composed of tiny, indestructible units called atoms. Democritus did not make any experiments about this hypothesis and had little to no evidence to support his theory, but his theory was kept alive.
2: Dalton | (1803) | He found that each chemical element is composed of unique types of atoms and they differ in masses. He devised chemical symbols and found the relative weight of each atom (Middle Picture). Dalton arranged each of these atoms in tables. The experiment that was used to discover this theory happened when he was studying meteorology. In one of his experiments, he found that evaporated water has an independent gas in the air. He was curious as to how water and air could be in the same space at the same time. He had performed a bunch of experiments to find out what would happen with the effect the individual gases on the whole mixture. While he was trying to figure this out, he had he had discovered the hypothesis that the sizes of particles making up different gases must be different. This lead to the discovery that each chemical element is composed of unique types of atoms and they differ in masses.
3: J.J. Thomson | (1897) | Thomson used cathode ray tubes in 3 different experiments and discovered an important part of the atom, which was the electron. In each experiment he came closer and closer to finding out that atoms are both positively and negatively charged. He also discovered the isotope. Thomson made a stream of ionized neon through a magnetic and an electric field and measured its deflection by placing a photographic plate in its path. Thomson observed two patches of light on the photographic plate which showed two different foundings of deflection.
4: Curie | (1898) | Curie discovered two new elements, polonium and radium. She did this by isolating these elements from a mixture called pitch blend. She developed ways to separate the elements. She tested everything to see if it was radioactive and was constantly working with this substance, needless to say, her and her husband both died from cancer. | Curie used to keep a bottle of Radium next to her bed, because she liked the glow it gave off.
5: Nagoaka | (1904) | Hantaro Nagaoka is credited with presenting the Saturnian model of the atom. In 1903, he proposed that electrons orbit a positive nucleus similar to the rings of the planet Saturn. Nagaoka rejected Thomson's model because opposite charges are impenetrable. He proposed an alternative model which a positively charged center is surrounded by a number of revolving electrons, in the manner of Saturn and its rings.
6: Millikan | (1909) | Millikan's "falling-drop method" allowed him to determine the charge carried by electrons. Then he proved that the experiment was correct for all electrons which demonstrated the atomic structure of electricity. He later proved his law of when things fall to the ground after entering the Earth's atmosphere. In the early 20s he worked mostly on ultraviolet and X-rays. Actually pulling down the ultraviolet spectrum to a place where no one else had taken it before and studies showed the radiations emitted from uranium and thorium and he named them alpha and beta. He had also determined the unit charge of the electron with his oil drop experiment at the University of Chicago. Which allowed the calculation of the mass of the electron and the positively charged atoms.
7: Rutherford | (1911) | Ernst Rutherford found 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 crazy proposal. 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 discovered the third atomic particle, the neutron.
8: Bohr | (1913) | Bohr worked under JJ Thomson and Rutherford while he was at Cambridge and there he was able to make some great discoveries. His best work was off of Rutherford's atom model. Bohr was able to create a more indepth model that is still used today to observe chemical and physical properties of different elements in the world.
9: H.J.G. Moseley | (1914) | Henry Moseley discovered that the energy of x-rays given off by the elements increased with each successive element in the periodic table. 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 elements. This new table left additional holes for elements that would soon be discovered. | "The atomic number of an element is equal to the number of protons in the nucleus"
10: Louis de Broglie | (1923) | He is noted both for his discovery of the wave nature of electrons and for his research on quantum theory. Einstein built on de Broglie's idea of "matter-waves"; based on this work, Erwin Schrdinger constructed the system of wave mechanics.
11: Erwin Schrodinger | (1926) | During his life he did numerous thing but he is most famous for his work on the Schrodinger Equation. He wrote the equation because he was dissatisfied with the quantum condition in Bohr's model of electron orbits and energy levels. He viewed electrons as continuous clouds and found "wave mechanics" as a mathematical model of the atom.
12: Chadwick | (1932) | Chadwick discovered the neutron. He used alpha particles to discover a neutral atomic particle with a mass close to a proton. Which ended up to be a neutron. In 1932, bombarded beryllium with alpha particles. He let the radiation given off by beryllium on a paraffin wax. It was found that protons were shot out form the paraffin wax. People began to look for what was in the "beryllium radiations".
13: Work Cited | "Atomic Structure Timeline." Atomic Structure Timeline. Web. 17 Oct. 2010.