4.01 Evolution of the Atomic Theory The Events Leading to the Discovery of the Building Block of Matter Dr. Fred Omega Garces Chemistry 152 Miramar College 1 401 Atomic Evolution
Idea of the Atom How did man come up with the idea of the atom? It took ~2400 years from when it was conceived to the time experimental evidence prove of the atom existence. 2 401 Atomic Evolution
History of Chemistry and development of the atomic model History of Atomic Structure The timeline of the progression of the development of the Modern Atomic theory. Atomic Theory Timeline Empedocles Aristotle Democritus Leucippus Zeno Aristotle Democritus Boyle Lavoisier Dalton J. Dalton J. Proust A. Lavoisier R. Boyle R. Bacon - 5 0 0-2 5 0 0 2 5 0 5 0 0 7 5 0 1 0 0 0 1 2 5 0 1 5 0 0 1 7 5 0 2 0 0 0 3 401 Atomic Evolution
Matter according to... Greek Philosophers 492-375 BC: Empedocles (supported by Aristotle) Earth-Air-Fire-Water 400 BC: Zeno (also supported by Aristotle) Matter can be infinitely divided 400 BC: Leucippus 470-375 BC: Democritus (student of Leucippus) Proposed the idea of the atom Matter -indivisible particles called atoms 4 401 Atomic Evolution
Next 2000 Years Alchemy (pseudo science) lead into gold experiments (black magic) took detailed notes (lab notebook conceived) Systematic metallurgy (extracted metals from ore) Roger Bacon (1214-1292) An English and alchemist, Bacon became known as the founder of experimental science. He believed that doing experiments for yourself rather than just accepting what other people tell you was the way to learn about nature. His most important work was the Opus Matus, in what he wrote about the scientific method of learning. He did many experiment which showed how rainbow are made by the effect of water drops on sunlight and how lenses could be used to help people with weak sight. 5 401 Atomic Evolution
Atomic Structure Revolution 1500 Robert Boyle The Skeptical Chemist He performed detailed experiment with gases; this initiated the break down of the Greek's model of matter. 1700 Antoine Lavoisier Father of modern chemistry (Law of conservation of mass) He made very careful measurements and concluded that mass is neither created nor destroyed. The thought at the time was then when a substance was burned, the ashes were lighter than the starting material, but Lavoisier did the experiment in an enclosed container and determined that the mass did not change. Subsequent experiments with other reactions was consistent with the conservation of mass findings. 6 401 Atomic Evolution
1790-1800 Joseph Proust (Law of definite proportion or Law of constant composition) A given compound always contain exactly the same proportion of elements by mass. i.e., H 2 O 2:16 or H : O 1 : 8 by mass always true whether there is 18 g or 139 g H 2 O For, H 2 O 2 2:32 the H : O is 1 g H : 16 g O In other words, Water found in SD or the Arctic or Jupiter s moon will all have 1 g hydrogen to 8 g oxygen ratios. Likewise glucose found in sugar cane or sweet beets have 6 g carbon to 1 g hydrogen to 8 gram oxygen ratios (12 x 6 C: 1 x 12 H: 16 x 6 O). i.e.,the relative numbers and kinds of atoms are constant. 7 401 Atomic Evolution
1800 John Dalton J. Dalton: Founder of the Modern Atomic Theory (Law of Multiple proportion) When two elements form a series of compounds, the ratio of the masses of the element that combine with 1 gram of the first element can be reduce to small whole numbers. Example: : Mass of nitrogen that combines with 1.0 gram of oxygen *Compound A 1.750 g g A/B = 1.750/0.8760 = 2 / 1 Compound B 0.8750 g g B/C = 0.8760 / 0.4375 = 2 / 1 Compound C 0.4375 g g A/C = 1.750 / 0.4375 = 4 / 1 Compounds consist of atoms which combines in whole number ratios. *N2O g N2O NO g NO NO2 g N1/2 O 8 401 Atomic Evolution
1808 Dalton's Atomic Theory : A new system of Chemical Philosophy All matter is made of atom. In a chemical reaction atoms are neither created or destroyed but only arranged differently. Atoms of the same element are identical. Atoms of different elements are not the same. Compounds are formed from two or more atoms. A given compound always has same relative number and type of atoms. 9 401 Atomic Evolution
... what s next?? Around this period, the model of the atom was gaining acceptance by scientist. Questions still unanswered - What is an atom? How is it put together? What are the parts of an atom? 10 401 Atomic Evolution
Early experiments to characterize the atoms The parts: Electrons - Thomson (1904), Millikan (1909) Protons - Goldstein (1886) Rutherford (1911) Neutrons - Sir James Chadwick (1932) 11 401 Atomic Evolution
Faraday Electrolysis ~1832 Contributions to the Modern Atomic Theory Balmer Discharge Tubes ~ 1830-90 Thomson e/m ~1897 Millikan e ~1909 Mendeleev Periodic Table ~1870 Becquerel, Curie Radioactivity α,β γ ~1897 Rutherford Nucleus ~1911 Michelson C ~1885 Planck Quantum ~1900 Einstein E=hv, E=mc 2 ~1905 The modern Atom: a flow chart of major theoretical and experimental contributions (who, what, and when) leading to our understanding of the atom. Aston Isotopes ~1909 Bohr Shell Model ~1913 Stern, Gerlach Electron Spin ~1921 Schrodinger Wave Model, Pauli Exclusion, Hund's Rules ~1926 Chadwick Neutron ~1932 d ebroglie Matter Waves ~1924 Heisenberg Uncertainty Principle ~1927 Scientific discovery is a complex human endeavor. The developments leading to our understanding of the modern atom, dramatically depict scientific discoveries not as individual efforts, but as the collective work of the scientific community. 12 401 Atomic Evolution
Early experiments to Characterize the atoms 1820's Sir Humphrey Davy / Michael Faraday Matter contains negative charge particles (electrons). Later, cathode rays were discovered and in the late 1800's cathode rays was investigated further. (1832) http://video.google.com/videoplay?docid=4144497206440839046 1900 John J. Thomson Cathode rays were electrically charge particles. Cathode Ray tube with electron stream where deflected by positive and negative plate: e / m = -1.76 10 8 C/g Proposed the "plum pudding model" theory of the atom; based on Coulomb's Law: F = k ( Q 1 Q 2 ) / r. Atoms contains protons and electrons. 1909 R.A. Millikan Oil drop expt.: Determine the electron s charge-value. e- = -1.60 10-19 C m = 9.11 10-31 kg 13 401 Atomic Evolution
Arrangement of subatomic particles 1911 Ernest Rutherford: Gold thin foil experiment. Reflection of alpha radiation on gold foil proved that the atom consisted of a very dense nucleus. Based on the particles deflection pattern, Rutherford calculated the fraction of the atomic volume occupied by the positive nucleus of about 1 part in 1014. The mass is so dense that a nucleus the size of a pea would have mass greater that 250 million tons. 14 401 Atomic Evolution
Characteristics of Atomic Parts The parts: Subatomic particles and their properties Particle Mass *Rel Mass (amu) Relative Charge e - ; electron 9.109 10-31 Kg 1/1837-1.602 10-19 C p + ; proton 1.673 10-27 Kg 1 + 1.602 10-19 C n; neutron 1.675 10-27 Kg 1 0 * Relative mass: 1 amu = 1 proton 15 401 Atomic Evolution
Summary These were a few of the experiments which lead to the discovery and a better understanding to the parts of the atom. Three sub-atomic particles : electron, protons, and neutrons combine in various numbers and arrangement to make the different 109 elements (and counting) in the periodic table. The picture of the atomic architecture is now complete (or is it?). From the time of Dalton g Thomson g Rutherford g Bohr g Schrodinger, our model of the atom has undergone many modifications. The model is not finish however. 16 401 Atomic Evolution
String Theory String theory is an active research framework in particle physics that attempts to reconcile quantum mechanics and general relativity general relativity. [1] It is a contender for the theory of everything (TOE), a manner of describing the known fundamental forces and matter in a mathematically complete system. The theory has yet to make testable experimental predictions, leading some scientists to claim that it cannot be considered a part of science. http://superstringtheory.com/. http://www.pbs.org/wgbh/nova/elegant/ 17 401 Atomic Evolution