The Structure of the Atom 3.2
Experiments Atom – the smallest part of an element that retains the chemical properties of that element. Subatomic particles – p +, n 0, e -
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)
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
The Electron Thompson experiment proves that atoms can be broken down and that e-’s exist. Has a large charge to mass ratio Charge = x couloumbs Mass x kg (1/1837 the mass of the simplest H atom)
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
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”
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
Atomic Nucleus Protons and neutrons Mass of a proton = x kg (quite heavier than an e-) Mass of neutron = x kg Nucleus differs in # of p + and n 0 which identifies that type of atom
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
Atomic Size Electron cloud (of – charge) Atomic radius ranges b/t 40 and 270 pm (1 pm = m) Atomic nuclei is about pm Very high mass to volume ratio = density of about 2x 10 8 metric tons/cm 3
Assignment Complete the Section Review on page 76 and do numbers 1-5. This is to be turned in by the end of class.