Introducing Atoms

Atomic Models The Greeks were the first people to think about the nature of matter. Democritus proposed the idea that matter is formed of small pieces that could not be cut into smaller parts. Atomos (uncuttable) or Atoms is what he came up with.

John Dalton John Dalton: an English chemist, inferred that atoms had certain characteristics. He came up with the Atomic Theory

Atomic Theory All elements are composed of atoms that cannot be divided. All atoms of the same element are exactly alike and have the same mass. Atoms of different elements are different and have different masses. An atom of one element cannot be changed into an atom of different elements. Atoms cannot be created or destroyed in any chemical change, only rearranged.

Atomic Theory Every compound is composed of atoms of different elements, combined in a specific ratio.

JJ Thomson (1897) Found that atoms contain negatively charged particles. Scientists knew that atoms themselves had no electrical charge. So, Thomas reasoned that atoms must also contain a positive charge. Discovered the electron (-) f-3 P. 103

Earnest Rutherford (1911) He was a student of JJ Thomas. Found evidence that countered Thomson’s model. Said that an atoms positive (+) charge must be clustered in a tiny region in the center called the nucleus. Protons contain a positive (+) charge

Bohr’s Model (1913) Student of both Thomson and Rutherford. His model said that electrons could have specific amounts of energy, leading them to move in certain orbits. Resemble the solar system.

Cloud of Electrons In the 1920’s the atomic model changed again. Scientists determined that electrons do not orbit the the nucleus like a planet. Electrons can be found anywhere in a cloud like region around the nucleus. An electrons movement is related to its energy level- specific amount of energy it has.