Chemistry Notes: The History of the Atom

•All matter is composed of elements•Element’s are made out of atoms that determine the elements’ physical and chemical properties.•Atoms are so tiny that they cannot be seen with the naked eye.

• The ideas about the structure of atoms changed throughout the centuries:• Scientists used indirect evidence to

determine how atoms were put together.• 1808 – John Dalton theorized that atoms

were solid masses containing no particles.• 1897 – J.J. Thompson suggested that the

electrons in atoms were scattered within a positively charged fluid.

• 1911 – Ernest Rutherford and H.G.J. Moseley proposed that atoms contained a nucleus of positively charged matter with electrons orbiting the nucleus.

• 1913 – Niels Bohr determined that electrons aren’t randomly located around the nucleus. Electrons move in specific layers or shells.

• Since 1926 - scientists’ understanding of an atom’s structure is that the nucleus contains protons and neutrons surrounded by “clouds” of electrons rather than electrons staying in shells.