history of atomic theory
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History of Atomic Theory. Atomic Theory Timeline. Democritus: 460-370 B.C. • 1. There are basic elements from which all matter is made • 2. Everything is made of small atoms moving in a void • 3. Some atoms are round, pointy, oily, have hooks, etc. to account for their properties - PowerPoint PPT PresentationTRANSCRIPT
History of Atomic Theory
Atomic Theory Timeline
Democritus: 460-370 B.C.
• 1. There are basic elements from which all matter is made
• 2. Everything is made of small atoms moving in a void
• 3. Some atoms are round, pointy, oily, have hooks, etc. to account for their properties
• 4. Ideas rejected by leading philosophers
John Dalton: 1766-1844
• 1. Each element is composed of extremely small indivisible particles called atoms
• 2. All the atoms of a given element are identical, but different from those of any other element
• 3. Atoms are neither created nor destroyed in any chemical reaction
• 4. A given compound always has the same relative numbers and kinds of Atoms. (Ex water is always H2O)
John Dalton: 1766-1844
• Where Dalton was wrong– 1. We can divide the atom• Subatomic particles include protons, neutrons,
& electrons• E = mc2 Atomic Bomb – 2. Not all atoms of a given element are
identical• Isotopes
J.J. Thomson: 1856-1940
• 1.Discovered electron in 1897 – Cathode Ray Tube Experiment – Electron has (-) charge: “e-” – Mass of e- = 9.11 x 10-28g• 1/1840 the mass of a proton– Protons discovered in 1886 by Goldstein• Proton has (+) charge: “p+” • Mass of p+ = 1.67 x 10-24g
J.J. Thomson: 1856-1940
• Cathode Ray Tube Experiment
• The beam Is attracted to (+) plate and repelled by (-) plate so it must be made of (-) charged particles!
J.J. Thomson: 1856-1940
• Thomson knew the atom as a whole was neutral so there must be (+) charged particles also.
J.J. Thomson: 1856-1940
• 2. Developed Plum Pudding model in 1904 – Atom is a sphere of (+) charge with (-)electrons
randomly scattered throughout – Modern version: “Choc. Chip Cookie” ?
J.J. Thomson: 1856-1940
• 3. Discovered isotopes in 1913– Different “versions” of the same atom. Atoms
of the same element, but with different masses.
• Nucleus and neutrons had not been discovered yet so he didn’t fully understand why isotopes existed
Ernest Rutherford: 1871-1937
• 1. Discovered Nucleus w/“Gold Foil Experiment” in 1909
– Bombarded a piece of gold foil (gold atoms) w/large, (+) charged alpha particles.
– Most passed right through but a few bounced back or were deflected at an angle
Ernest Rutherford: 1871-1937
• 2. Developed Nuclear Atom Theory in 1910– 1. The atom is mostly empty space– 2. In the center of the atom is a tiny nucleus,
which contains most of the atom’s mass– 3. The nucleus has a (+) charge – 4. Electrons are in afixed orbit in the empty
space around the nucleus
Ernest Rutherford: 1871-1937
James Chadwick: 1891-1974
• Discovered the neutron in 1932 – Neutron has no charge: “n0” – Mass of n0: 1.67 x 10-
24g
• Almost the same mass as a proton (both are much larger than the e-)
• Described the nucleus and explained isotopes discovered by JJ Thomson in 1913
– Isotopes of the same element have different masses (same # protons but different #s of neutrons)
Niels Bohr: 1885-1962
• Developed Planetary Model in 1913 • Nucleus is surrounded by electrons that are
orbiting in definite paths “like planets orbit the sun”
Niels Bohr: 1885-1962
• 2. Electrons exist in discrete “quantized” energy levels.
• • Quantized = the opposite of continuous, goes from one level to the next with nothing in between