Transcript Ch 2 Atomic History

Scientist: Joseph Proust, 1799
•
•
Modern Version of the
Experiment: Using
electricity, water is
electrolyzed into its
components, hydrogen
and oxygen.
Results: Water is always
found to contain
hydrogen and oxygen in a
mass ratio of 1:8 (volume
ratio of 2:1) no matter
how it is formed or where
it is found.
Scientist: John Dalton, 1808
•
Experiment: Wrote a book entitled: A New
System of Chemical Philosophy (1808)
•
Results:
1. All matter is composed of tiny indivisible
particles called atoms.
2.
Atoms of the same element are exactly the
same; atoms of different elements are
different.
3.
Atoms combine in whole number ratios to
form compounds. A given compound always
has the same relative number and kind of
atoms.
4.
Atoms cannot be created or destroyed in
chemical reactions, they are merely
rearranged.
Scientist: J.J. Thomson, 1897
• Experiment: An electric current
is passed through an evacuated
and sealed glass tube.
• Results: The voltage causes
negative particles to move from
the negative electrode to the
positive electrode. The path of
the electrons can be altered
(deflected) by the presence of a
magnetic field depending on the
applied magnetic and electric
fields.
Scientist: Robert Millikan, 1909
• Experiment: Oil drops are sprayed
above a positively charged plate
containing a small hole. As the oil
drops fall through the hole, they are
given a negative charge. Gravity forces
the drops downward. The applied
electric field forces the drops upward.
When a drop is perfectly balanced, the
weight of the drop is equal to the
electrostatic force of attraction
between the drop and the positive
plate.
• Results: Using this experiment,
Millikan determined the charge on the
electron to be 1.60  10-19 C.
Scientist: Henri Becquerel, 1896
•
Experiment: A radioactive substance is placed
in a shield containing a small hole so that a
beam of radiation is emitted from the hole. The
radiation is passed between two electrically
charged plates and detected.
•
Results: Three spots are noted on the detector:
– a spot in the direction of the positive plate,
– a spot which is not affected by the electric
field,
– a spot in the direction of the negative plate.
Scientist: Ernest Rutherford, 1910
• Experiment: A source of αparticles was placed at the
mouth of a circular detector.
The α -particles were shot
through a piece of gold foil.
•
Results: Most of the α particles went straight
through the foil without
deflection. Some α-particles
were deflected at high
angles.
Scientist: James Chadwick, 1932
•
Background: At the time, it was
well known that hydrogen had 1
proton, and helium had 2 protons,
but the ratio of the masses of a
hydrogen atom to a helium atom
is 1:4. Why?
•
Experiment: A thin sheet of
beryllium was bombarded with
alpha particles, and a very high
energy radiation similar to gamma
rays was emitted by the metal.
•
Results: The radiation was
electrically neutral. This radiation
was found to be another particle
which had no charge but a slightly
larger mass than a proton.