2. HISTORY OF THE ATOM MODERN THEORIES OF ATOMIC STRUCTURE

3. CATHODE RAY TUBE EXPERIMENT J.J. Thompson Conducted 3 attempts at experiment Determined that the ray was composed of a stream of negatively charged particles which he called electrons

4. PLUM PUDDING MODEL J.J. Thompson created it (1897) Atoms are big balls of positive charge with negative charges (called electrons) embedded in it – imagine a 3-D chocolate chip cookie. Discovered the negatively charged electron and predicted that there must be a positively charged particle to hold the electrons in place.

5. GOLD FOIL EXPERIMENT Ernest Rutherford (1911) Discovered the nucleus of an atom and named the positive particles in the nucleus “protons.” Electrons are scattered in empty space around the nucleus. How he explained these observations: –Positively charged particles would pass through the gold foil most of the time without being deflected. –The particles would only be deflected when they passed near the small positively charged nucleus of the atom (because like charges repel one another)

6. RUTHERFORD’S MODEL OF THE ATOM Nuclear Model: The positive charge is all concentrated in the middle of the atom (nucleues), while the negatively charged electrons float around throughout the rest of the atom’s empty space (electron cloud). Our current model of the atom says that the atom contains 3 types of subatomic particles that have the following properties: ParticleLocationChargeMass Proton (p + )Nucleus+11 amu Neutron (n 0 )Nucleus01 amu Electron (e - )Orbitals surrounding nucleus

7. WRITING ATOMIC SYMBOLS Atomic symbols have the general format: –A–A is the atomic mass (sometimes called the “mass number”) which is equal to protons + neutrons. Units of atomic mass are “amu.” –Z–Z is the atomic number. This is equal to the number of protons in all atoms and the number of electrons in neutral atoms. The atomic number determines the element. –X–X is the atomic symbol. If you match the number of protons to the atomic number on the periodic table, you can figure out what the symbol is supposed to be.