The Atomic Theory – p. 82-87 Democritus

1. The Atomic Theory – p. 82-87Democritus • Democritus, 440 B.C. • Thought matter was made of tiny particles • Believed these particles could not be cut into any smaller pieces. • Called this smallest particle, atom, which means not able to be divided

2. Dalton • British scientist • By the 1700’s scientists knew that elements combine to form different types of matter • Atoms looked like small, solid marbles • After extensive work, Dalton published his atomic theory in 1803 • All substances are made from atoms. Atoms cannot be created, divided, or destroyed • Atoms of the same element are exactly alike, and atoms of different elements are different • Atoms join with other atoms to make new substances

3. J.J. Thomson • 1897 – Made a correction to Dalton’s theory • Discovered that atoms contain negatively charged particles which he called electrons • Developed the plum pudding model to describe what he believed an atom to look like • This model had the electrons located throughout the atom

4. Rutherford • 1911- Rutherford revised atomic theory • Based on his experiments he proposed that atoms have a dense small center which he called a nucleus • This center contained positively charged particles which were called protons • Electrons surrounded the nucleus with large amounts of space between the nucleus and the electrons

5. Bohr • 1913 – Proposed that electrons move around the nucleus in certain pathways or energy levels

6. Modern Atomic Theory • Electron Cloud Model • Center of an atom very small and very dense. • Have positively charged particles called protons • Has particles called neutrons that have no charge • Electrons have a negative charge and move around the nucleus in orbits. These orbits do not have set paths so they look like a cloud around the nucleus