As with many scientific discoveries, this discovery raised more questions. Scientists had
long known about electric charge. Since most matter is neutral, they knew there must
be positive charges to balance the negative charge of the electrons. To put it at the
microscopic level: Atoms must contain positive charges to balance the negative
electrons. But how were these positive and negative charges arranged in an atom?
Answering that question is the topic of the next section.
Atoms
Contain a nucleus and one or more
electrons38.2 - Rutherford’s discovery of the nucleus
Nucleus: A relatively small region in the center
of an atom where the positive charge í and
most of the mass í of an atom is located.
The modern model of the atom describes it as a small positive nucleus, surrounded by
orbiting electrons. The radii of the orbits are far larger than the size of the nucleus. How
did scientists create this model of the atom? For instance, what led them to believe that
the nucleus was small compared to the size of the orbital radii?
Scientists in the early 20th century struggled to understand the nature of atoms. They
could not directly see the structure of atoms, but knew that they contained negatively-
charged electrons. They reasoned that atoms must contain equal amounts of positive
and negative charges since matter tends to be electrically neutral.
Without other data, most scientists thought that the positive charges that make up
matter were evenly distributed throughout the atom í why not? Since electrons have so
little mass, scientists knew that the positive charges carried almost all the mass of the
atom. Their mental picture of the atom looked like a blob of positively-charged cookie
dough with small chocolate chips (electrons) embedded in it. The model actually bore
the name of a more popular dessert at the time: The plum pudding model consisted of
negatively-charged electrons (plums) scattered throughout a massive cloud of positive
charge (pudding) that was distributed uniformly throughout the volume of the atom. This
model is shown in Concept 1.
Lord Rutherford made a major breakthrough in this area, winning the 1908 Nobel Prize
in Chemistry for his experiments. As a good scientist, he wanted data to support (or
contradict) the plum pudding model of the atom. At the time, Rutherford was studying
alpha particles, which are massive, positively charged particles that are emitted at high
speed from some radioactive substances (such as radon). He realized that a beam of
alpha particles might serve as a tool to probe the atomic interior.
In his experiment, he aimed a beam of these particles at a thin gold foil, and measured
the distribution of the outgoing alpha particles. This is known as a scattering experiment
í observing how the particles scatter í and it is a now-common technique to probe the
details of atomic-sized systems.
A mechanical analogy to what Rutherford did would be to probe the interior of a box by
shooting a high-powered BB gun at it. If the box were filled with sponge cake, the BBs
would pass through. If it had a metal plate inside, the BBs would rebound back. Or if
there were a small metal sphere inside, a small fraction would rebound or scatter
sideways, with a distribution of angles.
Rutherford initially assumed that the alpha particles would be passing through a
“pudding” of positive charges spread uniformly throughout the foil. Relying on this
model, Rutherford predicted that most of the alpha particles should just pass straight
through or be only minimally deflected.
This was not what he observed. Much to Rutherford’s surprise, a small fraction of the
particles were scattered by 90° or more. Occasionally, an alpha particle even
rebounded from the foil, straight back at the source. He concluded that the atom must
not have a uniform distribution of positive charge inside. Instead, the large force
necessary to cause such scattering of the positively charged alpha particle could be
Old “plum pudding” model
Positive charge distributed evenly, with
embedded electrons
·Wrong!Rutherford’s experiment
Fired alpha particles at gold foil
Some particles scattered by a large
amount
Implied compact, massive nucleusNucleus consists of positive charge