Atomic Concepts

1. Atomic Concepts I. Theory of the Atom A. Ancient Greece (2000+ years ago) Democritus Believed that matter could not be continuously divided Matter consists of small indivisible particles “Atom” = indestructible Particle are in continuous motion Four elements make up all matter and energy Earth, Wind, Water, Fire, No scientific evidence to show this.

2. B. Dalton’s Atomic Theory - 1803 John Dalton proposed the theory that: 1. All matter is composed of small particles which cannot be broken down (atoms) 2. All atoms of the same element are identical in size, mass and properties. Atoms of different elements are different in size, mass and properties 3.Atoms of different element combine in simple ratios to make compounds H2S PbO2 4. In chemical reactions, atoms are combined, separated, or rearranged (No atoms are created or destroyed) So at this point we believe that an atom is like a small solid ball of matter that cannot be split up

3. C. J.J. Thompson Experimented with cathode rays Used electric field to show that cathode rays are negatively charged particles All tested substances gave off the same negative particles Matter must be made of smaller particles, including the negative one After discovery of the proton, Thompson assumed an atom was a mixture of + and – charged particles, all mixed up • + - + - • + - • + - + Plum-Pudding Model

4. Screen Radioactive source Gold foil D.Rutherford Ernest Rutherford wanted to determine what an atom looked like. Fired + charged alpha rays at a very thin piece of gold (Gold foil) Put a screen behind the foil to determine what happens to the rays Most of the rays went straight through the foil Some rays deflected A very few rays came straight back

5. + - - - - - - Rutherford explained this by stating that a. Atoms have mostly empty space This is why most rays went straight through Rutherford model b. There must be a dense positive center to an atom This is why the positive rays deflected. (Positive rays are repelled by positive charged objects) Later experiments showed that electrons exist in the space between nuclei

6. E. Bohr Neils Bohr looked at the arrangement of electrons + 3 2 1 M L K e- e- nucleus - - - - - - Electrons exist in definite areas around the nucleus Energy levels e- e- Further from the nucleus, an electron had more energy Electrons can gain energy and “jump” to higher levels They can then give off the energy as they jump back down Bohr named the levels K,L,M,N,O… with K begin closest to the nucleus Now renamed 1,2,3,4… with 1 being closest to the nucleus and having the least amount of energy All atoms have the same types of energy levels Planetary model

7. Dr. Wave Mechanical F. Wave Mechanical Model Modified Bohr’s model Electrons are not in circular orbits, but exist in specific spaces around the nucleus Pattern is random, unpredictable nucleus level level Energy levels contain sublevels