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History of Atomic Theory

Who Was Right?. Greek society was slave basedNo experiments It was all a thought gameSettled disagreements by argumentAristotle was more famous so he wonHis ideas carried through to the middle ages.. John Dalton (Late 1700's). School teacher in EnglandBased his conclusions on experimentation and observations.Combined ideas of elements with that of atoms.

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History of Atomic Theory

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    1. History of Atomic Theory Democritus 460-371 B.C. ancient Greek philosopher believed all matter consisted of extremely small particles that could not be divided atoms, from Greek word atomos, means uncut or indivisible Aristotle believed all matter came from only four elementsearth, air, fire and water

    2. Who Was Right? Greek society was slave based No experiments It was all a thought game Settled disagreements by argument Aristotle was more famous so he won His ideas carried through to the middle ages.

    3. John Dalton (Late 1700s) School teacher in England Based his conclusions on experimentation and observations. Combined ideas of elements with that of atoms

    4. Daltons Atomic Theory All elements are composed of submicroscopic indivisible parts called atoms. Atoms of the same element are identical, those of different atoms are different. Atoms of different elements combine in whole number ratios to form compounds. Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.

    5. Parts of Atoms Most of Daltons theory is accepted today. Except the part about atoms being indivisible

    6. J.J. Thomson and the Cathode Ray Tube 1897

    7. Thomsons Experiment

    8. Thomsons Experiment

    9. Thomsons Experiment

    10. Thomsons Experiment Passing an electric current makes a beam appear to move from the negative to the positive end.

    11. Thomsons Experiment Passing an electric current makes a beam appear to move from the negative to the positive end.

    12. Thomsons Experiment By adding an electric field

    13. Thompsons Experiment By adding an electric field

    14. Thompsons Experiment By adding an electric field

    15. Thompsons Experiment By adding an electric field he found the moving particles were negative

    16. Thompsons Model Found the electron 1 unit of negative charge Mass 1/2000 of hydrogen atom Later refined by Millikan to 1/1840 Concluded that there must be a positive charge since atom was neutral Atom was like plum pudding A bunch of positive stuff, with electrons able to be removed.

    17. Other Pieces Proton positively charged pieces 1,840 times heavier than the electron Neutron no charge but the same mass as a proton.

    18. Ernest Rutherford Former student of J.J. Thomson Believed in plum pudding Wanted to find out how big they are Fired positively charged alpha particles at a piece of gold foil, which can be made a few atoms thick

    19. Rutherfords Experiment When alpha particles hit a flourescent screen it will glow. Heres what it looked like (pg. 90)

    21. What he expected to see

    22. Alpha particles should pass through without change in direction Positive charges were spread out evenly. Alone they were not enough to stop an alpha particle

    24. What he got

    25. How he explained it Atom is mostly empty Small dense, positive piece at the center Alpha particles are deflected if they get close enough to positive center

    26. Niels Bohr (1885-1862) Electrons have orbits about the nucleus (planetary theory) Electrons could only exist at given energy levels An energy level is where an electron is likely to be moving Energy levels were like steps on a ladder An electron can only be at any given step at any given time

    27. Modern Atomic Theory Bohr Modelshows electrons in orbit around protons and neutrons

    28. Structure of the Atom There are two regions The nucleus Protons and neutrons Positive charge Almost all of the mass Electron cloud Most of the volume of an atom Region where electron can be found

    29. Subatomic particles

    30. Counting the pieces Atomic number = number of protons Same as the number of electrons in a neutral atom Mass number = the number of protons + neutrons

    31. Atomic Mass Unit AMU Mass of a proton = 1.67 x 10 -27g A pretty inconvenient number New unit referenced to mass of an isotope of carbon: carbon -12 Carbon-12 has 6 protons and 6 neutrons Has a mass of 12.00000 amu an atomic mass unit Therefore 1 proton and 1 neutron has a mass of 1 amu.

    32. So why not whole numbers for atomic masses in periodic table? Reported numbers are average atomic mass units, reflecting the abundance of isotopes for any given number. In nature most elements occur as a mixture of two or more isotopes

    33. Isotopes Atoms of the same element can have different numbers of neutrons Different mass numbers Called isotopes

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