Unfortunately, Rutherford's
"planetary" model was still lacking in one
crucial aspect. Hydrogen atoms contain one
proton, while helium atoms contain two protons.
Thus, the ratio of the mass of a helium atom to
that of a hydrogen atom should be 2:1, but it is
actually 4:1. Rutherford and his colleagues
hypothesized there must be another subatomic
particle in the nucleus. James Chadwick provided
proof for the existence of this mysterious
particle. His experiment involved firing alpha
particles at a thin sheet of beryllium. This
metal emitted high-energy radiation similar to
gamma rays, and this radiation was shown to be
electrically neutral. The subatomic particles
associated with these rays were referred to as
neutrons, and they easily explained the mass
ratio mystery. Hydrogen atoms have one proton
and no neutrons, while helium atoms contain two
protons and two neutrons.
Hence, the
basic model of the atom consists of three
subatomic particles and a dense, central nucleus.
This nucleus is positively charged, contains
protons and neutrons, and is orbited by electrons.