In 1910, Ernest Rutherford
provided the next great leap in atomic theory
with his experiments, which involved firing
alpha particles at thin foils of gold. In
keeping with Thomson's "plum-pudding" model,
Rutherford, along with his associates Hans
Geiger and undergraduate Ernest Marsden,
expected most alpha particles to pass through
the foil with little to no deflection. The
results were somewhat shocking. Although most
particles passed with little deflection, some
deflected at large angles. In some cases, they
rebounded in the opposite direction! Rutherford
was quoted as saying, "It was as incredible as
if you had fired a 15-inch shell at a piece of
tissue paper and it came back and hit you."
Rutherford arrived at two
different conclusions. First, atoms must be mostly
empty space. Second, the atom's positive charges
must be concentrated in a dense, central nucleus,
which is the core of the atom. Rutherford named
these positively charged particles protons. He
then proposed the "planetary" model of the atom,
which stated that atoms have a dense, positively
charged nucleus orbited by negatively charged
particles.