A. Democritus (Greek, 460-370 BC) — thought all matter was composed of indivisible particles called "atoms"

B. John Dalton (English, 1766-1844) — stated specific rules about the composition of matter (first Atomic Theory);

He also developed the first atomic model — Solid Sphere Model — a solid, indivisible sphere.

C. JJ Thomson (1897) — discovered electron using a cathode ray tube; changed model to the Plum Pudding Model where negative electrons float in positive "pudding".

D. Robert Millikin (1909) — measured charge and mass of electron doing the Oil Drop Experiment

E. Ernest Rutherford (1909) — did the Gold Foil Experiment where he discovered the nucleus- a small, dense, positively charged area in the center of the atom; he developed the Planetary Model where the nucleus is in the center and the electrons orbited around the nucleus like planets around the sun

F. Neils Bohr (1913) — altered Rutherford’s model because he measured energy levels of electrons, so he claimed that electrons traveled in circular orbits of specific energy values around the nucleus. It is called the Bohr Model.

G. Erwin Schroedinger (1926) — a mathematician who developed a mathematical model explaining the atomic model. It is called the Electron Cloud Model because the electrons are in a probability area around the nucleus. These electrons are in energy levels of various shapes, so where the electron cloud is most dense, there exists the greatest probability of finding an electron.

I. 1981 — Scanning Tunneling Microscope (STM) used to see atoms for the first time

each lepton has an anti-particle with the same mass but opposite charge; the anti-particle of the electron is the positron

B. Hadrons — this family has hundreds of particles (we will learn two)

1. protons

2. neutrons

hadrons interact with a strong force due to the nucleus; leptons are only weakly bound together in cloud; so electrons are involved in chemical reactions while protons and neutrons are involved in nuclear reactions; nuclear reactions are more powerful because the strong force has to be broken

A. Murray Gellman and George Zweig (1964) discovered quarks — these are subatomic particles which make up protons and neutrons (electrons can’t be broken apart further as far as we know today)

B. Up quarks have a +2/3 charge while down quarks have —1/3 charge; as a result, the proton is made up of 2 up and 1 down quark to result in +1 charge; the neutron is made of 1 up and 2 down quarks to result in zero charge.

B. Atomic Number (Z) — number of protons; also number of electrons if there is no charge

C. Mass Number (A) — the number of protons plus neutrons in a specific nucleus

D. Atomic Mass — the weighted average mass of all isotopes of an element

E. Ion — an element with a charge

F. Isotope — an element with same number of protons, but different numbers of neutrons