The Atomic Theory – p Democritus  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

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

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

Rutherford  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

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

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