Download History of the Atom

How we came to understand the basic building block of
matter.
 Matter is continuous.
 Basic elements: Earth, Water, Air and Fire
 Democritus (470-370):
“Matter is not infinitely
divisible. Matter is
discrete.”
 Atomos – indivisible.
 Aristotle (484-322 B.C.):
“Empty space cannot
exist. All matter is
continuous.”
 Aristotle’s views go unchallenged for 2000 years.
 Early Scientists approach matter
empirically.
 Robert Boyle reintroduces early
ideas of atoms, molecules, and
compounds.
 Joseph Priestly isolates “oxygen”
in air.
 Antoine Lavoisier documents,
through many experiments, the
law of conservation of mass.
 John Dalton proposes an explanation.
 The smallest particle of
an element that retains
the properties of an
element.
 The scanning tunneling
microscope allows for
individual atoms to be
seen.
 The Cathode Ray Tube
 Thomson measures the effects of magnetic and
electric fields on the cathode ray.
 When an electric charge is applied, a ray of radiation
travels from positive side to negative side.
 Particles in the ray are attracted to a positively charged
plate
 The smallest atom can be separated into positive and negative particles.
 Thomson calls these particles “corpuscles.” Name is later changed to the
“electron.”
 Thomson is able to determine the mass to charge ratio of the negative
electrons.
 Millikan uses the oil-drop apparatus to determine the
charge of a single electron.
 With Thomson’s charge to mass ratio, Millikan is able
to calculate the mass of a single electron.
 Ernest Rutherford
discovers “Alpha
particles,” positively
charged particle
radiation.
 He experiments with the
interaction of “alpha
particles” with solid
matter.
 Most alpha particles went through the foil, BUT, a few
were deflected or even bounced back.
 Rutherford concluded that atoms are mostly empty space.
 Almost all of the atom’s positive charge is contained in a
dense region in the center of the atom, the “nucleus.”
 Repulsion between the positive nucleus, and the positive
alpha particles caused some deflections.
 Rutherford refines the
model to include a positively
charged nucleus.
 He proposes that the
nucleus is made of larger
positively charged particles
called “protons”. Electrons
are found outside the
nucleus.
 A few years later, a
colleague, James Chadwick,
discovers that neutral
particles, “neutrons” are also
in the nucleus.