Ernest Rutherford
• Student of J.J. Thomson (1894)
Early studies based on radioactivity
• Discovered that some materials emit radiation
 Alpha particle: Eventually was confirmed to be helium nuclei, which
meant an alpha particle was simply 2 protons and 2 neutrons
 Like Thomson, Rutherford was interested in the deflection patterns of alpha
particles when exposed to electric & magnetic fields
 Found the deflection patterns by measuring alpha particles position on
photographic film
 By accident, he noticed that if the alpha particles passed through a thin sheet
of mica, the images on the film were blurred
Rutherford and alpha particles
• The images were sharp if the mica was not
present
 Something about the mica sheet was causing the alpha
particles to scatter at seemingly random small angles,
resulting in blurred images
Rutherford & Geiger
• Wanted to study the effects of alpha particles with
matter
 Had to develop a way to count individual alpha particles when they hit the screen
 Found that a screen coated with zinc sulfide emitted a flash of light each time it was hit
by an alpha particle
 Rutherford & Geiger had to sit in a dark room for hours and individually count the
flashes of light
Rutherford asked Geiger to measure the angles of deflection
when the alpha particles were passed through a thin (.00004 cm)
sheet of gold foil
 When the gold sheet was bombarded with alpha particles, Geiger found that the
scattering was small
 Results were consistent with Rutherford’s expectations. Because he knew that alpha
particles had a considerable mass and moved quite rapidly, he anticipated that virtually
all of the alpha particles would go through the metal foil without much disruption
Rutherford, Geiger, & Marsden
Marsden was a graduate student working in Rutherford’s
lab and Geiger had suggested that a research project should
be given to Marsden
 Rutherford: “why not let him see whether any alpha particles can be
scattered through a large angle?”
 Marsden had found that a small fraction (perhaps 1 in 20,000)
of the alpha particles were scattered through angles larger
than 90 degrees
Rutherford was in awe
 “It was quite the most incredible event that has ever happened to me in
my life. It was almost as incredible as if you fired a 15-inch shell at a piece
of tissue paper and it came back and hit you!”
What could the results from this gold
foil experiment mean?
The + charge in an atom of gold was known, but
if this charge were collected on a sphere the size
of an atom (like Thomson suggested), the
repulsion would be far too weak
 To explain the force experienced by the alpha particle, the charge and
much of the mass would have to be collected in a much smaller sphere
 Published results in 1911 and proposed a model for the atom that is still
accepted today
 All of the positive charge and essentially of the mass of the atom is
concentrated in an incredibly small fraction of the total volume of the
atom, which he called the nucleus (Latin for “little nut”)
Further findings from Rutherford’s
experiment
• Most of the alpha particles were able to pass right through
• A small fraction came close to the nucleus of a gold atom as
they passed through and were slightly deflected from the
positive-positive repulsion of the alpha particle and nucleus
• But occasionally, an alpha particle would run directly into the
nucleus and would result in a great repulsion that deflected
the alpha particle through an angle of 90 degrees or more
 By carefully measuring the fraction of the alpha particles deflected through large
angles, Rutherford was able to estimate the size of the nucleus (JUST LIKE WE
DID!)
 Found that the radius of the nucleus is at least 10,000 times smaller than the
radius of the atom
 The vast majority of the atom is therefore empty space!!
Someone throw out the plum
pudding!!
Rutherford revised Thomson’s plum pudding model,
showing how electrons could orbit a positively charged
nucleus, like planets orbiting a sun
Because the majority of the “plum-pudding”
atom would be electrically neutral (no charge),
the alpha particles would have no problem
shooting through
Rutherford’s atom
• Nuclear Model
Modern version of Rutherford's experiment