THE RUTHERFORD MODEL OF THE ATOM
Around 1900, the atom was just considered a small bunch of positively charged
“stuff” embedded with negatively charged electrons. This theoretical structure was
known as the raisin pudding model; the pudding was the positively charged part of
the atom and the raisins were the electrons. However, laboratory experiments from
1909 to 1911 led Ernest Rutherford to propose a radical revision of this model.
Rutherford fired alpha particles (α), which were known to be relatively massive
and to carry an electric charge of +2e, at an extremely thin sheet of gold foil. An
alpha particle consists of two protons and two neutrons, tightly bound together. If
the atom is really just a glob of positive charge dotted with tiny negative electrons,
then the heavy alpha particles should sail right through the target atoms, with little
deviation.
For the most part, this is what the experiments revealed. However, a small
percentage of the alpha particles exhibited behavior that was completely
unexpected: Some were deflected through very large angles (90° to 180°). This
was explained by postulating that the positive charge of the atom was not spread