1. The Structure of the Atom 3.2

2. Experiments  Atom – the smallest part of an element that retains the chemical properties of that element.  Subatomic particles – p +, n 0, e -

3. The Electron  First discoveries – late 1800’s  Pass electric current thru a tube of gas at low pres. called “cathode ray tube.”  Opposite side glowed – thought to be b/c of cathode rays and traveled from cathode (-) to the anode (+)  Rays were deflected from magnetic field just like wire carrying ele. current (which are known to be negative)

4. The Electron  JJ Thompson  Experiments in 1897 measured ratio of charge particles to their mass  Same for all gas or metal combo  Conclusion = cath. rays are composed of all the same particles called electrons  Cathode Ray Tube Video Cathode Ray Tube Video

5. The Electron  Thompson experiment proves that atoms can be broken down and that e-’s exist.  Has a large charge to mass ratio  Charge = -1.602 x 10 -19 couloumbs  Mass 9.109 x 10 -31 kg (1/1837 the mass of the simplest H atom)

6. Inferences from Experiment  Atoms are electrically neutral = there must be a + charge  There must be other particles b/c of weight  Model proposed = plum pudding

7. Atomic Nucleus  Rutherford’s gold foil experiment  Bombard with α particles (+ charge and 4x mass of H atom)  Some particles were deflected (1 in 8000)  “as if you had fired a 15-inch artillery shell at a piece of tissue paper and it came back and hit you”

8. Atomic Nucleus  Conclusion  There must be some force within the atom that deflected the particles.  There is a densely packed mass with a + charge  Very small nucleus – like a marble in a football stadium  Gold Foil Experiment Gold Foil Experiment

9. Atomic Nucleus  Protons and neutrons  Mass of a proton = 1.673 x 10 -27 kg (quite heavier than an e-)  Mass of neutron = 1.675 x 10 -27 kg  Nucleus differs in # of p + and n 0 which identifies that type of atom

10. Atomic Nucleus  Like repels like, but  Up to 83 protons can exist together in a stable nucleus  Nuclear forces – short range forces that hold the particles of the nucleus together

11. Atomic Size  Electron cloud (of – charge)  Atomic radius ranges b/t 40 and 270 pm (1 pm = 10 -12 m)  Atomic nuclei is about 0.001 pm  Very high mass to volume ratio = density of about 2x 10 8 metric tons/cm 3

12. Assignment  Complete the Section Review on page 76 and do numbers 1-5. This is to be turned in by the end of class.