History of the Atom

Democritus

460

http://plato.stanford.edu/entries/democritus/ He said that the atom was a small indivisible particle.

Dalton

Jan 1, 1803

http://www.rsc.org/chemsoc/timeline/pages/1803.html Atoms cannot be divided. Atoms combine in small whole ratios to form compounds. Atoms of the same element are identical in mass. Atoms of different elements have different masses.

JJ Thomson Cathode Ray Experiment

Oct 12, 1895

http://www.aip.org/history/electron/jj1897.htm
A cathode ray is a tube with electrons traveling in a straight line emitting a green colored light. What he did was place two cylinders with a slit in them. They were connected with an electrometer. The electrometer was dectecting elctric currents and negative charge when he neared a magnet. The electric current and the ray reacted to the magnet. He discovered the electrons.

JJ Thomson Plum Pudding Model

Oct 12, 1906

http://www-outreach.phy.cam.ac.uk/camphy/nucleus/nucleus1_1.htm
http://www.universetoday.com/38326/plum-pudding-model/
The protons (positive charge) is the whole and the electrons (negative charge) are the droplets.

Robert Millikan Oil Drop Experiment

Oct 13, 1909

http://chemistry.about.com/od/electronicstructure/a/millikan-oil-drop-experiment.htm
There were two metal plates sepatated by a insulating material. He sprayed droplets of oil from the top and watched them fall in the botton through a microscope. An electric field was active in the experiment. The bottom was ionized. With voltage the droplets rised which meant the interaction and reaction of radiation, depending on the droplet's mass. What he discovered was the electrons' real value and charge.

Ernest Rutherford's Gold Foil Experiment

Oct 14, 1911

http://www.rsc.org/chemsoc/timeline/pages/1911.html
He shot radioactive through thin metal gold foils and tracks them using screens coated with zinc sulfide. Most particles were shot at another direction once they came near the center.

Rutherford's Model of the Atom

Oct 15, 1911

http://www.britannica.com/EBchecked/topic/514258/Rutherford-atomic-model
He suggested that the atom had its nucleus or center and then the space around it is just empty space. Then, a cloud of electrons surround the atom.

Niehls Bohr's Model of the Atom

Oct 14, 1915

http://csep10.phys.utk.edu/astr162/lect/light/bohr.html
http://www.ccmr.cornell.edu/education/ask/index.html?quid=126
The model is the nucleus at the center and at least three rings surrounding the nucleus. Each ring contains different number of electrons. It is sometimes called the planetary model, because it looks like the sun and the planets circling around the "sun." Electrons are attracted to them, but they can't slow down. They are forced to orbit te nucleus for eternity.

James Chadwick

Oct 14, 1932

He discovered the neutrons.

Aristotle

Mar 2, 2022

http://www.iep.utm.edu/aristotl/ He believed that matter was continuous. That all elements contained earth, fire, water, and air.