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Atomic Theory

Atomic Theory. Ancient and Modern. Ancient Theory. Democritus. Greek philosopher/scientist Stated that everything is made up of “void” in which exists infinite, tiny, indivisible particles “Atom” comes from Gr. “atomon,” meaning “indivisible”. John Dalton (1766-1844). 5 basic postulates:.

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Atomic Theory

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  1. Atomic Theory Ancient and Modern

  2. Ancient Theory Democritus • Greek philosopher/scientist • Stated that everything is made up of “void” in which exists infinite, tiny, indivisible particles • “Atom” comes from Gr. “atomon,” meaning “indivisible”

  3. John Dalton (1766-1844) 5 basic postulates: • All matter consists of tiny particles called atoms • Atoms are indestructible and unchangeable • All atoms of the same element have the same weight, and all atoms of different elements have different weights* • Atoms in reactions combine in simple, whole-number ratios (Law of Definite Proportions) • Sometimes atoms combine in more than one simple, whole-number ratio *We now say that atoms of the same element have the same “nuclear charge”

  4. J.J. Thomson (1856-1940) • “Cathode ray” (stream of electrons) experiment • “Plum-pudding” model of the atom • Mass-to-charge ratio of electron J.J. Thomson’s cathode ray apparatus—the negatively-charged “cathode rays” (electrons) are attracted to the positively-charged plate

  5. Ernest Rutherford (1871-1937) • Discovered nucleus with gold foil experiment • Shot He atoms at a piece of gold foil surrounded by radioactive Zn—if plum-pudding model was correct, atoms should go straight through • Found that some atoms ricocheted! • Concluded that atoms have a dense, positive core (nucleus)

  6. Rutherford’s Experiment

  7. Max Planck (1858-1947) • Looked at light emitted by heated materials • Came up with an equation that related energy emitted to the frequency of light emitted E=h f h=constant (6.626 x 10-34 J s) f=frequency (Hz) E=energy (J) • Discovered that energy can only be emitted in discrete packets, called “quanta” (sing. “quantum”)

  8. Whee!! 4 3 2 1 Niels Bohr (1885-1962) • Postulated that atoms have different energy levels • Theorized that electrons can absorb photons of light and “jump” from one energy level to the next • When electrons fall back, they emit photons of light Energy level H atom

  9. Quantum Theories • Each electron in an atom has 4 quantum numbers to define position and properties: • Principal quantum number (n)—describes e- energy level: n=1,2,3… • Angular momentum quantum number(l)—describes orbital:l=0for s, 1 for p, 2 for d, etc. • Magnetic quantum number (ml)—describes position (i.e. which “box” in orbital diagrams):ml= -lto +l • Spin quantum number (ms)—describes movement: -½, +½ • Pauli Exclusion Principle: no two electrons can have the same 4 quantum numbers

  10. More Quantum Stuff Chromium orbital-filling diagram ml=-1 ml=0 ml=1 ml=0 S orbital: n=1, l=0 S orbital: n=2, l=0 P orbital: n=2, l=1 S orbital: n=2, l=0 P orbital: n=3, l=1 D orbital: n=3, l=2 n=3, l=2, ml=-2, ms=+½

  11. Yet More Quantum Stuff • Albert Einstein (1879-1955): Light has properties of both particles and waves. • Louis de Broglie (1892-1987): Matter has wave-like properties. • Heisenberg Uncertainty Principle: It is impossible to know both speed and position of an electron at the same time.

  12. Bibliography www.britannica.com/ nobel/cap/oruthef002a4.html http://www.bookrags.com/sciences/chemistry/atomic-theory-woc.html

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