4.2. Structure of the Atom http://www.ck12.org
FIGURE 4.9
Rutherford’s gold foil experiment.portion of the alpha particles could be better explained by an atom that contained a very small, densenucleus.
Because some of the alpha particles emitted from the source were repelled by the nucleus, Rutherford concluded
that the nucleus must be made up of these positively charged alpha particles, which he namedprotons. He proposed
that atoms consist of a small, positively charged nucleus surrounded by negatively charged electrons, as shown in
theFigure4.10.
FIGURE 4.10
Rutherford’s atomic model.Bohr’s Atomic Model
In 1913, shortly after Rutherford’s work on the nucleus, Neils Bohr proposed what became known as a planetary
model of the atom. Bohr’s model was based upon the work done by Max Planck and Albert Einstein, who at the time
were studying quantum theory which looks at the energy associated with matter. The planetary model was useful
for relating atomic structure to the wavelengths of light that an element emits when heated. Bohr’s model, as well as
the work of Planck and Einstein, will be discussed in the chapterElectrons in Atoms.
Discovery of the Neutron (1932)
In 1932, James Chadwick discovered the neutron. Chadwick was an English physicist who was mentored by
Rutherford. His experiment consisted of bombarding beryllium atoms with alpha particles through a paraffin wax