Transcript The Structure of the Atom

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+, n0, 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
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)
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
Atomic Nucleus
 Protons
and neutrons
 Mass of a proton = 1.673 x 10-27kg
(quite heavier than an e-)
 Mass of neutron = 1.675 x 10-27 kg
 Nucleus differs in # of p+ and n0
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 = 10-12 m)
 Atomic nuclei is about 0.001 pm
 Very high mass to volume ratio =
density of about 2x 108 metric
tons/cm3
Assignment
 Complete
the Section Review on
page 76 and do numbers 1-5. This is
to be turned in by the end of class.