Models of the Atom a Historical Perspective

Greek: 400 B.C. - Democritus thought matter could not be divided indefinitely. He named the smallest piece of matter “atomos,” meaning “not to be cut.” Democritus

Greek: 350 B.C - Aristotle changed an earlier theory that matter was made of four “elements”: earth, fire, water, air. Aristotle was wrong. However, his theory persisted for 2000 years. Aristotle fire air water earth

John Dalton (Atomic Theory) 1800 -Dalton proposed a modern atomic model based on experimentation not on pure reason. All matter is made of atoms. Atoms of an element are identical. Each element has different atoms. Atoms of different elements combine in constant ratios to form compounds. Atoms are rearranged in reactions. His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios).

1897- J.J. Thomson (The electron) “Plumb Pudding Model” Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding.

1897- J.J. Thomson Thomson studied the passage of an electric current through a gas. As the current passed through the gas, it gave off rays of negatively charged particles. https://www.youtube.com/watch?v=O9Goyscbazk

1908- Ernest Rutherford (nucleus) Rutherford shot alpha () particles at gold foil. Zinc sulfide screen Thin gold foil Lead block Radioactive substance path of invisible -particles Most particles passed through. So, atoms are mostly empty. Some positive -particles deflected or bounced back! Thus, a “nucleus” is positive & holds most of an atom’s mass.

Rutherford Scatting Experiment https://www.youtube.com/watch?v=XBqHkraf8iE