Seeing Is Believing
One of the major advances in determining atomic structure came with the
creation of a cathode ray tube (CRT). A CRT is essentially an electron gun—a
device capable of producing electrons and shooting them through a vacuum,
along with a screen that is able to detect the electrons.
Scientists originally determined the rays created by a CRT were negatively
charged, as they were attracted to positive objects and repelled from negative
ones. The first scientist to determine the rays were actually made up of particles
was J.J. Thomson, who also determined the relative mass and charge of the
particles. These particles later came to be known as electrons.
The most famous experiment that helped shape modern atomic theory was
conducted by Ernest Rutherford. Rutherford shot a beam of helium atoms at a
sheet of gold foil. The vast majority of atoms passed through undeflected, which
was expected—atoms were considered to be mostly empty space, a theory that is
still held to be true today. However, a very small percentage (<1%) of the atoms
were deflected by the foil. Rutherford theorized that this was due to the presence
of a highly dense nucleus located in the center of the atom and that had sufficient
mass to deflect the helium atoms.
The theory of the atom continues to evolve over time to expand beyond an
understanding of the basic subatomic particles, but that will be addressed further
in Chapter 6.