Atomic Theories

Democritus All matter is made up of “uncuttable” chunks of matter. 460-370 BC Atoms are solid, homogenous, indestructible, and indivisible. All matter is made up of “uncuttable” chunks of matter. Atomos = Greek for uncuttable Atoms! Different kinds of atoms have different shapes and sizes. Size, shape, and movement of atoms determines the properties of matter.

Earth Water Wind Fire Aristotle 384-322 BC All matter is made of: Was a philosopher… did not use experimental science

Dalton’s Atomic Theory John Dalton proposed an atomic theory matter is composed, indivisible particles (atoms). all atoms of a particular element are identical different elements have different atoms atoms combine in certain whole-number ratios to form compounds In a chemical reaction, atoms are merely rearranged to form new compounds; they are not created, destroyed, or changed into atoms of any other elements.

Experiments led him to the discovery of ELECTRONS. J.J. Thompson 1856-1940 Studied how a beam of energy traveling through a cathode ray tube moved in response to a magnet. The gas used was neutral, and yet the ray had a negative charge proving that atoms could be broken. Thought of atoms as electrons floating in positive material. Experiments led him to the discovery of ELECTRONS. The beam itself was made of electrons.

Cathode–Ray Tube

Thompson’s Model (Plum Pudding or Cookie Dough)

The Gold Foil Experiment Rutherford 1849-1919 The Gold Foil Experiment Shot beam of positive particles at layer of gold foil. Almost ALL went right through. Some scattered. This meant atoms are mostly empty space, with a small dense core (nucleus) and Electrons scattered around it. (Nuclear Model)

Bohr 1885-1962 Observed that light would sometimes be absorbed by atoms at one wavelength, and emitted at another. This led to him concluding that electrons travelled in varying orbits around the nucleus.