The Development of Atomic Theory Chapter 4.1 Notes

The Beginnings First theory of atoms was proposed more than 2,000 years ago Democritus: 4 th century BC, Greek philosopher who suggested that the universe was made of indivisible units Called the units atoms From the Greek word atomos, meaning “unable to be cut or divided” Could not prove his theory

Dalton’s Theory John Dalton: English schoolteacher who proposed a revision of the atomic theory in 1808 All atoms of a given element were exactly alike, and atoms of different elements could join to form compounds Dalton based his theory on experimental evidence (Democritus had no experimental evidence for his theory) Parts of his theory could not explain all of the experimental evidence found—atomic theory changed over time as more scientists continued to do experiments and find more information

Law of Definite Proportions A chemical compound always contains the same elements in exactly the same proportions by weight or mass H 2 O, CO 2 Always 2 hydrogen for every 1 oxygen; always 1 carbon for every 2 oxygen This was evidence for Dalton’s theory that atoms of different elements could combine to form compounds

Thomson’s Model J.J. Thomson: British scientist who conducted an experiment suggesting that atoms could be divided (1897) He had been studying cathode rays (rays in vacuum tubes) Cathode rays were made of negatively charged particles that came from inside atoms; therefore atoms could be divided into smaller parts

Thomson discovered electrons, negatively charged particles inside the atom

Plum-Pudding Model New model of the atom based on Thomson’s findings Electrons are spread throughout the atom

Rutherford’s Model Ernest Rutherford: another British scientist that developed an experiment to test Thomson’s model Proposed that most of the mass of the atom was concentrated at the atom’s center

Rutherford’s Experiment Rutherford's gold-foil experiment Experiment suggested that an atom’s positive charge is concentrated at the center of the atom—i.e. nucleus Positively charged particles aimed at the nucleus were deflected

Niels Bohr Proposed that electrons move around the nucleus in specific layers, or shells Every atom has a specific number of electron shells

James Chadwick Worked with Rutherford to determine the subatomic particle with no charge He called this particle the neutron Neutrons are also found in the nucleus

Modern Model of the Atom Modern model of the atom has electrons moving around the nucleus in a cloud It is impossible to predict the exact location or pathway of an electron at any given point in time