Atomic Theory The History of the Atom

Democritus (400 B.C.) Greek Philosopher…..named the atom no science back then. so no controlled experiments Idea #1..matter can not be cut forever…. There is a smallest piece of everything. That smallest piece is called an atom. In Greek atom means indivisible

Democritus defines atoms Smallest piece of matter All atoms made of same thing just different shapes and sizes Atoms are always moving Atoms can join together with other atoms

Lavoisier (1798) Scientist! Established Law of Conservation of Mass = matter can not be created nor destroyed Matter is made of atoms so ….. atoms can not be created or destroyed Law but only under normal Earth conditions

Dalton (1803) Scientist! Assembled first Atomic Theory Based on many facts determined by many experiments 1) all matter is made of atoms 2) atoms are indestructible 3) all atoms of same element are the same 4) all atoms of different elements are different 5) joining atoms of different elements makes compounds

Thomson (1897) Plum pudding model Named electrons Know his experiment

Thomson’s Experiment 1) Thomson invented the cathode ray tube http://www.youtube.com/watch?v=IdTxGJjA4Jw&feature=related thompson video 1) Thomson invented the cathode ray tube http://www.youtube.com/watch?v=5NwMPcYH71g&feature=related ray tube production

2) When metal at the end of the tube was heated particles shot out of it and down the tube 3) He knew the particles were doing this because when they hit the phosphorous at the end of the tube it glowed making the beam visible

4) Thomson saw that he could bend the beam of particles by running an electrical current through the tube 5) The beam bent toward the positive electrode …….so the particles must be… Negative

Stuck in positive dough 6) The atoms of metal that the particles came from were neutral …..so…… There must be something positive in the atom to balance out the negative charge of the particles Plum pudding model Negative electrons Stuck in positive dough http://www.youtube.com/watch?v=IdTxGJjA4Jw&feature=related thomson video

Rutherford (1911) Geiger counter….measures radioactivity Alpha particles :positive particles given off by some radoiactive materials http://www.youtube.com/watch?v=wzALbzTdnc8&NR=1

Rutherford’s discoveries: Solar system or planetary model of atomic theory Discovered protons Later helped discovered neutrons (1932)

Rutherford’s experiment 1) Shot positive particles at gold foil 2) Detector behind foil flashed where the particles hit. 3) If Thomson’s model was correct most particles should go straight through http://www.youtube.com/watch?v=Zd6_zVdMgJk

Rutherford’s Results 1) Most particles did go straight through This supported Rutherford’s idea that atoms are …. Mostly empty space 2) A few particles bounced back, this supported… The idea that there is a small hard positive core 3) A few were slightly deflected showing that they… were pulled off track by nearby electrons.

Rutherford & Chadwick Found that the protons + the electrons did not add up to the mass of the atom Hypotheses were proposed to explain this mystery mass Most popular was gamma ray hypothesis until……

Chadwick (1932) Disproved gamma ray hypothesis Found neutral particles about same mass as a proton Particles were neutral so named Neutrons New atomic model included protons and neutrons in nucleus

Chadwick’s formula

Bohr Model Electron’s distance from the nucleus determined by how much energy they have Farther out = More energy

Electrons can change energy levels To move up an energy level they must ……. gain a set amount of energy To move down an energy level they must……. give off a set amount of energy. http://www.youtube.com/watch?v=45KGS1Ro-sc&feature=related The energy give off is a packet of light energy called a photon. http://www.youtube.com/watch?v=-YYBCNQnYNM&feature=related

Wave model: Quantum Theory of Atoms Electrons must stay a set distance from the nucleus How far away depends on how much energy they have More energy = Electron could be anywhere around the atom at that distance http://www.youtube.com/watch?v=f64EYPIily4&feature=related

Up for debate??