Atomic Theory http://videos.howstuffworks.com/discovery/29430-assignment-discovery-origin-of-the-atom-theory-video.htm Model A familiar idea used to explain unfamiliar facts observed in nature Theory An explanation of observable facts and phenomena To remain valid, models and theories must: Explain all known facts Enable scientist to make correct predictions Episode 301

Proposed the existence of an atom Democritus Proposed the existence of an atom Word comes from the Greek word atomis which means not to cut or “indivisible” Aristotle Rejected the idea of the atom Said matter could not be cut continually Episode 301

John Dalton’s Theory proposed that atoms Atoms are the building blocks of matter Atoms are indivisible Atoms of the same element are identical Atoms of different atoms are different Atoms unite in small whole number ratios to form compounds Episode 301

J. J. Thomson Credited with discovery of electron: a blow to Dalton’s indivisible atom Cathode Ray – negatively charged stream of particles (electrons) Proposed the plum pudding model of the atom: negatively charged electrons embedded in a ball of positive charge http://videos.howstuffworks.com/science-channel/29292-100-greatest-discoveries-the-cathode-ray-tube-video.htm Episode 301

Ernest Rutherford’s Gold-foil Experiment http://www.youtube.com/watch?v=XBqHkraf8iE Aimed alpha particles at gold foil Most passed through A few particle were deflected Some particles were even bounced back Conclusions to Rutherford’s experiment Most of the atom is empty space Dense positively charged nucleus Planetary model Episode 301