History of the Atom

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First and Last Name

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Period

John Dalton

• Proved that atoms exist by determining that gases combine with each other in specific ratios: only particles can do that!
• Believed atoms were indivisible, simple solid spheres

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Dimitri Mendeleev

• Developed the Periodic Table of Elements with blank spaces for elements yet to be discovered: genius!!
• His predictions led to the discovery of many elements

Marie Curie

• Discovered the elements radium and polonium, and coined the term “radiation”
• She suggested that atoms were made of smaller parts; hypothesized about radioactive decay

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JJ Thomson

• Created an experiment using gases in a cathode ray tube

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• This resulted in

the discovery

of the electron

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• Plum Pudding

Model

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JJ Thomson

• Realized that atoms are made of smaller parts

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• Discovered the electron has a very small mass

Plum Pudding

Ernest Rutherford

• Shot radioactive alpha particles through a thin piece of gold foil

Ernest Rutherford

• In his famous “gold foil” experiment, he proved that the electron is far from the center of the atom
• Discovered that atoms have a dense, positively charged nucleus

Niels Bohr

• Proposed that electrons orbit in rings called energy levels, like planets around the sun
• Chemical properties of atoms are determined only by electrons in the outermost energy level

James Chadwick

• Discovered the neutron, including its mass
• Worked with Oppenheimer in America on the “Manhattan Project,” realizing that the development of an atomic weapon was inevitable

Many Scientists

Today

• Today evidence suggests that electrons are not like pinpointed planets orbiting the sun, but rather like clouds of negative charge surrounding the nucleus.

Many Scientists

Today