2. Democritus: @ 400 B.C. Theorized that if you were to cut something in half, then cut it in half again and again… Eventually you would be left with something too small to be cut. Therefore, Democritus gave us “atomos,” or “uncuttable,” to be the word for the smallest individual particle of a substance.

3. John Dalton: 1808 Dalton’s Atomic Model –All matter is composed of atoms. –Atoms of each element are the same size, mass, etc., but are different from atoms of other elements. –Atoms can’t be subdivided (cut) or destroyed. Envisioned atoms as “tiny, solid balls”

4. Cathode Ray Tube If you run electricity through a gas filled tube, you see a stream of particles How do we know they are particles? –A cross left a “shadow”, so something must be traveling in a straight line. How do we know it isn’t light? –J.J. Thomson put a magnet next to the tube and it bent the beam. Light is not magnetic! Beam had a negative charge

5. J.J. Thomson: 1897 Thomson’s Atomic Model Discovered negatively charged electrons. Envisioned atoms as a “positively charged sphere with electrons embedded in it.” (plum pudding) “Muffin” Model Positively charged bread Negatively charged “blueberries” floating around (electrons )

6. Hantaro Nagaoka: 1904 Nagaoka’s Atomic Model Proposed a nucleus Proposed a nucleus Envisioned electrons orbiting a positively charged sphere in the center. Envisioned electrons orbiting a positively charged sphere in the center. “Planetary” Model “Planetary” Model Electrons = planets Electrons = planets Positive Center Sphere = Sun Positive Center Sphere = Sun

7. Alpha Particle Cannon Shoots fast-moving positively charged particles (result of radioactive decay) Thomson’s model predicts that alpha particles shot at a very thin piece of gold foil will pass right through it like a bullet through tissue paper.

8. Ernest Rutherford Designed the gold foil investigation Was shocked to discover “back- scattering” of a small percentage of alpha particles Like shooting a machine gun at tissue paper and every now and then one of the bullets bounces back Thomson’s model could not explain the results of the alpha particle and gold foil test Instead, the data supported the idea of a very small, very dense, positively-charged nucleus

9. Ernest Rutherford: 1911 Rutherford’s Atomic Model The atom is composed of mostly empty space. Small, dense, positively charged nucleus containing almost all of the atom’s mass. “Peach” Model Peach Pit = Nucleus Meat = Empty space with electrons randomly orbiting nucleus.

11. Niels Bohr: 1913 Bohr’s Atomic Model Electrons do not “randomly” orbit the nucleus. Electrons move in specific “layers” or “energy levels” / “shells” (3-dimensional). Atoms absorb or give off energy when electrons move from one “level” to another. Bohr’s Model “onion” model Electrons move on “layers”.

12. Valence Electrons Valence electrons are the electrons located furthest from the nucleus They are a determining factor in chemical bonding. If you visualize the atom like Bohr (an onion), the valence electrons are the electrons located on the outer layer.

13. A Problem with Atomic Theory

15. Electron Cloud Model: Modern (refined since 1920’s) Electrons form a negatively charged cloud around the nucleus. It is impossible to determine exactly where an electron is at any given time. (probability instead) Higher probability of electrons near the nucleus.