History of the Atom

1. History of the Atom

2. Democritus (400 B.C): • proposed that matter was composed of tiny indivisible particles, which he called atomos. • The word atoms comes from the Greek word atomos.

3. Aristotle • Said that all substances are composed of four elements: Earth, Wind, Fire, and Water. His theory prevailed for 1000+ years. • This was incorrect.

4. John Dalton (1808) • An English schoolteacher, John Dalton proposed a theory called the atomic theory. It stated: • 1. Every element is made of tiny, unique particles called atoms that cannot be subdivided. • 2. Atoms of the same element are exactly alike. • 3. Atoms of different elements can join to form molecules.

5. John Dalton, continued.. • Less than 100 years after Dalton published his atomic theory, scientists determined that atoms can be broken down even further.

6. Dmitri Mendeleev (1869) • Published the first periodic table, grouping elements by common physical and chemical properties.

7. J.J. Thompson (1897) • Discoverer of the electron. • Explained cathode rays as streams of electrons. • Proposed the first model of the atom, which described electrons like “raisins in a pudding of positive charge”.

8. Marie Curie (1898) • Coined the term radioactivity. • Won the 1903 Nobel prize for her discovery of radioactivity. • Won the 1911 Nobel prize for discovering the elements radium and polonium.

9. Ernest Rutherford (1909) • Discovered the existence of the atomic nucleus. • Discovers the proton in 1914.

10. Neils Bohr (1913) • At age 28, the physicist publishes the theory of electron shells. This leads to the modern theory of the atom and quantum mechanics. • Awarded the 1922 Nobel Prize in physics for discovering the structure of the atom. • Bohr’s models are the illustrations of atoms commonly used today.

11. Neils Bohr

12. James Chadwick (1932) • Discovers the neutron. Finally all three subatomic particles are discovered.

13. Lise Meitner (1938) • Discovers evidence of unusual radioactive behavior of uranium, which leads to the discovery of nuclear fission.

14. Linus Pauling (1939) • Published a book that shows how Bohr’s model of the electron shell explains chemical bonding. • This book provides the explanation for the structure of the periodic table. • He received the Nobel Prize in chemistry in 1954, and the Nobel Peace Prize in 1962.

15. Enrico Fermi (1942) • Creates the first man made nuclear chain reaction in a laboratory, as part of the Manhattan Project.

16. Albert Einstein (1917) • Proposed the idea that atoms can be stimulated to produce radiation. • Excitation of an atom causes it to release energy as a photon. • This is the conceptual basis for laser technology.

17. J. Robert Oppenheimer • Scientific director of the Manhattan Project, to develop the first atomic bomb. • Known as the “father of the atomic bomb”