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4.1 Studying Atoms Ancient Greek Models Democritus (2500 yrs ago) he said everything is made of small particles He called them atoms from atomos “uncut” or “indivisible” (undevideable) He thought there were different types of atoms with different properties Aristotle thought there was no limit to the number of times atoms could be divided. Though wrong, this was the thinking for a long time. Dalton’s Atomic Theory 1800’s Worked with gases in the air Noted that compounds have fixed composition All matter is made of small particles called atoms that cannot be divided 1. all elements made of atoms 2. all elements of same element have the same mass 3. compounds contain atoms of more than one element 4. atoms of different elements combine in same way in a particular compound Dalton DEPICTED ATOMS AS SOLID SPHERES Later, his theory was revised as new evidence was found. Thomson’s Model 1856-1940 Used electric current to study atoms (Review transparencies of experiment) Stated the charged particles from his experiment came from within atoms This was the first evidence that atoms were made of smaller particles. Model stated atoms are neutral, but both positive and negative charges are present. Positively charged mass and negative charges flying around Charges and mass evenly spread out (like chocolate chip ice cream). Rutherford’s Atomic Theory 1899 Discovered “alpha particles” - fast moving particles with positive charge. His “Gold Foil experiment” found the path of an alpha particle passing through a piece of gold foil Most went through foil undeflected, or deflected only a tiny bit A few were deflected a lot About one in 20,000 was deflected more than 900 A very tiny number bounced straight back like they had struck something He thought that since the alpha particles were positively charged, they must be being deflected by a positive charge within the atom. He theorized that: All atoms have positive charge concentrated in nucleus