History of the Atom

John Dalton

• Proved that atoms exist
• Believed atoms were indivisible, simple solid spheres

Dmitri Mendeleev

• Developed the Periodic Table of Elements
• His predictions led to the discovery of many elements

Marie Curie

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

J.J. Thomson

• Realized that atoms are made of smaller parts
• Discovered the electron
• Discovered the electron has a very small mass

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

• Discovered 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 “shell”

J. 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

Modern Scientists

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