History
of Atomic Structure
1) Democritus - Greek philosopher
a) matter is composed of tiny particles called atoms which
could not be subdivided or made any smaller
b) atoms were different only in
size and shape
c) atoms were in constant motion
d) atoms were able to join
with other atoms to form different types of matter
2) John Dalton click on name
for more complete background
a) matter is composed of tiny particles called atoms
b)
atoms of a particular element are alike in size, shape, and weight but
differ from atoms of other elements
c) during chemical changes, atoms of
different elements unite forming molecules (compounds)
d) during these
chemical changes, atoms themselves do not change, that is, are not broken
down
e) when atoms combine, they do so in definite whole number ratios by
weight (called the Law of Definite Proportions)
3) J. J. Thomson - gas tube experiments of the late 1800's
a) when a high electrical voltage was placed across
electrodes in a glass vessel from which most of the air had been removed an
electrical discharge occurred
b) these 'cathode rays' traveled in straight
lines because they cast a well defined shadow on any fluorescent surface of
objects placed in their path
c) since these `rays' could be deflected by
magnetic and electrical fields they consisted of electrical charges in
motion
d) from the direction of deflection the charge on these particles had
to be negative
e) in 1897 J. J. Thomson first measured the ratio of charge
to mass, e/m, of these small mass, negatively charged, high velocity
particles called electrons
4) Wilhelm Roentgen
a) found that shooting a beam of high speed electrons at a
glass plate, by using a cathode ray tube, it gave off a type of radiation he
called X rays
b) medical uses for this technique were soon developed
5) Henri Becquerel - 1896
a) performed the following experiment in attempting to find
out if fluorescence contained X rays
1) exposed photographic
film wrapped in black paper to fluorescent chemicals in bright sunlight
2)
he knew X rays would expose the film as well as sunlight
3) he tried to
determine in sunlight might cause fluorescence which would be seen in
exposed film
4) found that a fluorescent chemical, potassium uranyl sulfate,
fogged the film, suggesting X rays might be present
b) due to cloudy weather
he left this set-up in a drawer for several days
c) he developed the
film, hoping that residual fluorescence might produce a slight fogging
d) he
found a strongly fogged film (sunlight had not been needed to cause exposure
of the film)
e) this new type radiation was not X rays because it needed no
cathode ray tube and could not be turned off and was not fluorescence
because it did not need sunlight or particle or any type radiant energy to
cause it to work
f) the study of this new radiation (called radioactivity by
Marie Curie) helped to change the nineteenth-century concept of atomic
structure
6) Marie and Pierre Curie - 1898
a) both studied radioactive elements in detail
b) discovered
that thorium was radioactive and discovered (and named) polonium and radium
7) Radioactivity (click to see
full information)
8) Rutherford-Geiger-Marsden gold foil experiments - 1911
a) these men knew that alpha particles were, charged helium
ions that traveled in straight lines at very
high speeds
b) the plum pudding model of the atom (J. J.
Thomson - 1897) said atom was sphere of positive matter in which electrons
were randomly embedded (like raisins)
c) according to the plum pudding model
the expected results of shooting a beam of alpha particles at thin gold foil
should be that the alpha particles would pass straight through since there
would be no concentration of charge and mass large enough to deflect the
relatively massive alpha particle
d) actual results: most of the particles
passed through unharmed (missed Au atoms) a few were deflected off to the
side a very few were reflected backward (head on collision with nucleus
e) initial conclusions reached:
1) scattering must be due to encounters with charged atomic
particles that are much smaller than the atom and at least as heavy as an
alpha particle
2) must be some positive electric charges inside the atom to
compensate for negative charges of the electrons
3) must be something in the
atom much heavier than an electron to account for the mass of the atom
4) atoms must be mostly
empty space
5) suppose an atom has a small central core (nucleus) which
contains most of the mass of the atom and carries a positive electric charge
that attracts the negative electrons (helping to keep them in orbit around
this nucleus)
f) final conclusions:
1) positively charged
protons concentrated in region inside atom called nucleus
2) atoms total diameter
much larger than that of the nucleus
3) negatively charged particles, called electrons, were
circling in orbit around the nucleus (a balance of forces holds them in
orbit; the positive nucleus pulls electrons inward toward nucleus and
a counterbalancing force called centripetal (which is the force that tends
to make rotating bodies move away from the center of rotation) tending to
throw them outward (like a ball on a string being swung around your head)).
4) the positive electrical charge of the proton is equal in strength to the
negative electrical charge of the electron
5) the weight of the proton is
1836 times as great as the weight of an electron
6) a particle in the
nucleus having no electrical charge was predicted
9) James Chadwick - 1932
a) found the weight of atom did not check out when working
with isotopes and thus found the predicted particles in nucleus with nor
charge called the neutron
b)
neutron weighs about the same as a proton
c) neutron has no electrical
charge
DEFINITIONS:
a) radiation - energy transferred by electromagnetic waves
b) fluorescence -
visible light given off by a substance when struck by radiation or electrons
c) Law of Definite
Proportions - atoms combine in whole number ratios based on weights
d) Law of Electrical
Charges - like charges repel, unlike charges attract
e) radioactivity -
spontaneous release of radiation and particles by unstable nucleus
