Timeline Use your textbook and any other resources available to research the contributions and approximate dates of the following scientists/ models: Democritus.

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Timeline Use your textbook and any other resources available to research the contributions and approximate dates of the following scientists/ models: Democritus John Dalton JJ Thomson Ernest Rutherford Niels Bohr Electron cloud model

Development of Atomic Theory

Democritus First proposed atomic theory in the 4th century BC The universe is made of units called atoms (Greek for “unable to be cut or divided”) Did not have enough evidence to convince others that his theory was true

Democritus

Dalton Atoms cannot be divided 1808 Revised atomic theory Atoms cannot be divided All atoms of a given element are exactly alike Atoms of different elements join to form compounds

Dalton Experimental evidence Foundation of modern atomic theory Law of definite proportions: a chemical compound always contains the same elements in the same proportions Foundation of modern atomic theory

Law of Definite Proportions Fill in the following table: To calculate percent: divide mass of element by mass of compound. Example: C=12 g, O=3 g Percent C = 12/15 = .8 = 80% Element 1 Element 2 Sample ID Name Percent 1001 1002 1003 1004 1005 1006

Thomson 1897 Cathode ray experiment He found that cathode rays were made of negatively charged particles from inside atoms This showed that atoms could be divided into smaller parts! (electrons)

Cathode-ray Tube Experiment

Plum Pudding Model Electrons (negatively charged) are spread throughout the atom

Rutherford 1899 Atoms have a dense, positively charged nucleus Developed his theory based on the gold-foil experiment New model of the atom has electrons orbiting the positive nucleus

Gold-foil Experiment

“It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

Bohr model 1913 Electrons only in certain energy levels (or shells) Each level holds a certain number of electrons

Electron Cloud Model Developed in 1925 by Heisenberg and Schrodinger Bohr’s model could not explain all electron behavior Shows movement of electrons

James Chadwick 1932 Discovered the neutron by comparing proton mass to atomic mass

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