ATOMIC THEORY

History of the Atom

http://www.youtube.com/watch?v=njGz69B_pUg&feature=related

John Dalton

http://www.youtube.com/watch?v=wsYDL6EjV4k

Suggested that elements were made up of atoms.

A given compound always contains the same combination of atoms.

Prepared the first table of atomic masses. Law of Multiple Proportions: When 2 elements

form a series of compounds (i.e., CO and CO2), the ratios of the masses of the second element that combine with 1 g of the first element can always be reduced to small whole numbers. (See Sample Exercise 2.1, p.45)

Amadeo Avogadro

Avogadro’s Hypothesis: at the same temperature and pressure, equal volumes of different gases contain the same number of particles.

J.J. Thomson

http://www.youtube.com/watch?v=_SY7qBrVxsU&feature=related

Cathode-ray tubes (cathode = negative electrode)

The ray emanating from the cathode was repelled by the negative pole of an applied electrical field. So what did Thomson propose the ray was composed of?

Determined the charge-to-mass ratio of an electron: c/m = -1.76 x 108 C/g

Thomson (cont.)

Since electrons could be produced from electrodes of various metals, all atoms must contain electrons, so they must have some positive charge.

Postulated that an atom consisted of a diffuse cloud of positive charge with the negative electrons embedded randomly around it. (Plum Pudding Model)

Robert Millikan

Oil Drop Experiment: http://www.youtube.com/watch?v=91E6KvCvRf0

Altered the voltage on charged plates to stop the fall of charged oil drops.

Using the voltage and the mass of the oil drop, he was able to determine the charge on an oil drop.

Using the charge on the oil drop and Thomson’s charge-to-mass ratio, Millikan calculated the mass of an electron as 9.11 x 10-31 kg.

Ernest Rutherford

Gold Foil Experiment: http://www.youtube.com/watch?v=mHcjiiP0HB4&feature=related

Tested Thomson’s plum pudding model by directing α particles at a thin sheet of gold foil.

Expected result: particles would pass through

Actual result: most passed through, but some were deflected and reflected

These results indicated a concentration of positive charge (nucleus), with electrons moving around it at some distance.

Niels Bohr

Quantum Model: http://www.youtube.com/watch?v=Cn6v5ygyZHQ

The electron in a hydrogen atom moves around the nucleus only in certain allowed circular orbits (much like the planetary model)

Bohr also postulated that atoms can become “excited” by an electrical current, absorbing energy and then releasing that energy in the form of light, with each element emitting certain, unique wavelengths (or spectrum).

http://www.youtube.com/watch?v=xUX-nmPL9SE&NR=1

Schrodinger

Said electrons were more like waves than particles

James Chadwick

Discovered the neutron This accounted for the previously

unexplained difference between an element’s atomic number and its atomic mass

http://www.youtube.com/watch?v=HnmEI94URK8&feature=related

Werner Heisenberg

Discovered both Bohr AND Schrodinger were correct: an electron is a particle with wave-like properties

Photon: a “particle” of electromagnetic radiation

Also said you cannot know both an electron’s location AND its spin at the same time.

Heisenberg’s Uncertainty Principle: http://www.youtube.com/watch?v=Fqr4MFg6GHI&feature=related

Atomic Structure