1. Atomic Structure
Chp 4

2. Philosophers “thought” about atoms
Democritus
B.C.
-matter made of indivisible, indestructible particles called “atoms” that can
move through empty space
Aristotle
B.C
-matter made of earth, air, wind, and fire. No empty space
-His popularity replaced Democritus’ ideas

3. Experiments test matter
John Dalton- 1800’s
-Law of Conservation of Mass Experiments
-Dalton’s Atomic Theory
1. elements made of indivisible atoms
2. atoms of element are identical and different from other elements
3. atoms combine in whole # ratios
4. atoms rearrange/join in chemical reactions
“Marble Model”

4. Experiments with Cathode Rays
Crookes-1800’s
-observed cathode ray particles always bent away from
the (-) side of a magnet and towards the (+) side of a magnet.
-Thomson-1897
-calculated this particle's mass was 1/1840th of a hydrogen
atom. He discovered the first subatomic particle called the electron.

5. Experiments continue with the electron
Millikan-1910’s
The Oil Drop Experiment
-electrons that stuck to the oil drops had charges that varied by a
common denominator of x10-19 Coulombs.
-a single electron carries a charge of 1-
-Millikan used the electron's charge/mass to determine the mass of an electron.
-a single electron has a mass of 9.11x10-28 g.

6. A new model of the atom was needed
Called the Plum Pudding Model
-solid positive sphere
-electrons scattered randomly inside
-overall charge of the atom is neutral

7. Experiments continue with the atom
Rutherford-1911
Gold Foil Experiment
-observed alpha particles deflected at large angles. Rutherford concluded that
the atom is mostly space and most of its(+) charge and mass is contained
in the nucleus
-Atom's are neutral due to (+) charge of the nucleus balancing the (-) charge of
the electrons.
-Rutherford's model could not account for all of the atom's mass

8. A new model of the atom was needed

9. More subatomic particles are discovered
-Rutherford concluded in 1920 that the nucleus contained (+) particles called protons.
-Chadwick concluded in 1935 that the nucleus contained another subatomic particle called the neutron. The proton and neutron are nearly equal in mass.

10. Particles smaller than protons, electrons, and neutrons discovered
1964-Zweig and Gell-Mann proposed quarks independently.
1968-Experiments showed quarks scattering out of colliding protons.

11. The Large Hadron Collider

13. Dear Democritus
Atomic Theory Infographic