Transcript The Atom
The Atom You think you know, but do you??? Objectives • Understand the models of Democritus, Dalton, Thomson, Rutherford and Chadwick • Compare and contrast the atomic models of Democritus and Dalton • Define an atom • Distinguish between the subatomic particles in terms of relative charge and mass • Describe the structure of the nuclear atom, including the locations of subatomic particles There are a lot of depictions out there! Which are correct? Video: Nova Atoms Podcast Definition of an Atom • Smallest particle of an element that retains the properties of that element • All matter is made of atoms History of the Atom • Democritus, 400 B.C. – Greek philosopher – Universe is made of invisible units called atomos – Matter is predominantly empty space – Atoms differ in their size, shape and movement – Changes in matter result from changes in groupings of atoms, and not changes in the atoms themselves Atom is derived from the Greek word meaning “unable to be divided” DEMOCRITUS’ ATOM (400 BC) WOULD HAVE LOOKED LIKE THIS: (draw it!) DEMOCRITUS’ ATOM (400 BC) WOULD HAVE LOOKED LIKE THIS: History of the Atom • John Dalton, 1808 – English schoolteacher – Atomic Theory built on Democritus’ Theory of Atomism • Every element is made of tiny particles that cannot be subdivided (atoms) • Atoms of the same element are identical • Atoms cannot be created, divided, or destroyed • Atoms combine in whole number ratios to form molecules • Chemical reactions are atoms being separated, combined, or rearranged DALTON’S ATOM (1808) WOULD HAVE LOOKED LIKE THIS: DALTON’S ATOM (1808) WOULD HAVE LOOKED LIKE THIS: Atoms are particles of specific elements. J.J. Thomson, 1897 – Found that all matter contained negative particles – Cathode Ray Tube experiments William Crookes pioneered cathode ray tube design Thomson investigated properties of the rays in detail Cathode Ray Tube Experiment It did not matter what element he got the cathode ray tube from; the cathode rays were identical every time. Open file: Cathode Ray Tube.flv http://www.chem.uiuc.edu/clcwebsite/video/Cath.mov Thomson’s Model •Found the mass of an electron by passing electricity through gases •Mass of an electron was about 1/2000th that of a hydrogen atom •Contradicted Dalton – Atoms could be divided •Attempted to explain why matter is not always charged THOMSON’S ATOM (1904) WOULD HAVE LOOKED LIKE THIS: THOMSON’S ATOM (1904) WOULD HAVE LOOKED LIKE THIS: Plum Pudding Model: + negative particles embedded in a base of positive charges to form uncharges atoms. (this accounts for the fact that matter is usually neutral) Thomson’s Plum Pudding Model Ernest Rutherford, 1909 • Gold foil experiment Video: Gold foil experiment reenactment! Open file: gold foil reenactment.mp4 RUTHERFORD’S ATOM (1909) WOULD HAVE LOOKED LIKE THIS: RUTHERFORD’S ATOM (1909) WOULD HAVE LOOKED LIKE THIS: The Nuclear Atom: electron area with tiny, dense, positive nucleus; mostly empty space James Chadwick, 1932 • A student of Rutherford • “extra mass” problem of isotopes… CHADWICK’S ATOM WOULD HAVE LOOKED LIKE THIS: Why were these last particles so tough to detect??? CHADWICK’S ATOM WOULD HAVE LOOKED LIKE THIS: The Nuclear Atom: electron area with tiny, dense, positive nucleus; nucleus includes neutrons, atomic number equals number of protons, mostly empty space Disclaimer….