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CHEMISTRY NOTES Atomic Structure The Development of the Atomic Model • • • • • The following men each contributed to the Atomic model Democritus John Dalton J.J. Thompson Ernest Rutherford Neils Bohr Early Ideas of Matter • • • Matter could be divided in half again and again infinitely. Leucippus (ancient greek philosopher – 400 BC) originated the concept that matter can only be divided to a point then it losses its properties. Leucippus student was Democritus. Democritus • Democritus coined the term atom Atom is from Greek – • the prefix "a" means "not" and the word "tomos" means cut. Our word atom therefore comes from atomos, a Greek word meaning uncuttable. Democritus came about this idea by way of thought not experiment. Democritus Main Points • • • • • All matter is composed of atoms, which are bits of matter too small to be seen. These atoms CANNOT be further split into smaller portions. There is a void, which is empty space between atoms. Atoms are completely solid. (WRONG) Atoms are homogeneous, with no internal structure. (WRONG) Atoms are different in size and shape John Dalton • • • An English schoolteacher Father of the present day atomic model Dalton's idea of an element is what we believe today – – an element is a chemical substance that cannot be decomposed further by chemical means • Lived 1766-1844 Daltons Main Points • • • • Chemical elements are made of atoms The atoms of an element are identical in their masses (wrong) Atoms of different elements have different masses Atoms only combine in small, whole number ratios such as 1:1, 1:2, 2:3 and so on. (Law of Definite Proportions) – From Proust’s observation Evidence for The Atomic Theory Dalton’s Evidence • Two elements can combine in different ways to form more that one compound. The masses of elements in those compounds are still found in fixed, small whole # ratios – Law of Multiple Proportions • Carbon monoxide / carbon dioxide – Inference different compounds can be made from the same elements, just using different amounts havingsmall, whole-number ratios, again, means there must be a basic unit of matter (atoms) that combines to form compounds. The Law of Definite Proportions • • States: A given chemical compound always contains the same proportion by mass of its constituent elements. Another way to say it: the relative amount of each element in a particular compound is always the same, regardless of preparation or source. The Law of Definite Proportions • • Water will always have 2 atoms of hydrogen and 1 atom of oxygen – no matter how you make it or where it comes from or how much of it you have. H 2O – 2 atoms H X 1 amu = 2 amu – 1 atoms O X 16 amu = 16 amu • • 11.11% of water will always be hydrogen 88.89% of water will always be oxygen J.J. Thompson • • • English Physicists Lived 1856-1940 In 1897, J.J. Thomson discovered the electron, the first subatomic particle J.J. Thompson Main Points • • • Negatively Charges electrons exist embedded in a positively charged substance that completely surrounds them. (WRONG) The positive material balances out the negative charges on the electrons so that the atom is neutral Under certain conditions electrons could be removed from the atom. J.J. Thompson’s Atomic Model Ernest Rutherford • • • Professor at Cambridge University Discovered the nucleus of the atom Lived 1871-1931 Rutherford’s Experiment • • • Rutherford worked with alpha particles Alpha particles are basically a helium nucleus (i.e. 2 protons and 2 neutrons) Alpha particles are emitted from radioactive substances Rutherford’s Experiment • • Procedure: Rutherford shot alpha particles at a thin gold foil. Result: Most of the particles went straight through the gold foil. However some of the particles were deflected, some were even reflected – According to the Thompson model this should not happen. Rutherford’s Experiment Rutherford’s Experiment Conclusion: • Most of the alpha particle going strait through the gold foil must mean that most of the atom is empty space. • According to the Thompson model the atom should not have enough density or electrical charge to repel an alpha particle moving at 10% the speed of light. Rutherford’s Experiment Rutherford’s Experiment • Rutherford concluded: most of the mass of an atom and all its positive charge lie in the very small very dense region. – This region was called the nucleus. – Rutherford named the small, positively charged particles in the atom protons. Rutherford’s Experiment • Calculations show that the nucleus is 1/100,000th the size of the entire atom, however over 99% of that atoms mass is in the nucleus.