How many times can one cut a piece of gold in half? A Greek philosopher
named Democritus was the first in a long line of scientists and mathematicians to try to
answer this very difficult question. Around 450 BC, Democritus stated that all mater was
composed of particles that he called atomos (where the English word atom comes from)
and that these particles could not be cut. This went against the present day theory
of Aristotle which was that matter could be cut infinitely. Unfortunately for Democritus,
his theory was not accepted in his lifetime.
John Dalton came up with an atomic theory of matter in 1803. In his theory he
stated four theories. 1.)All mater is composed of atoms 2.) Atoms of same elements are
the same 3.) Atoms of different elements are different and 4.) Atoms unite in definite
whole number ratios when they form compounds. Though his theory has some errors,
these theories are the basis for our understanding of atoms and chemistry.
One mistake of Dalton was his belief that atoms were round and hard. This became
the primary ambition for scientists after Dalton, to find out the structure of an atom. In
1897, Joseph John Thomson discovered electrons. Using a tube, magnets and charged
plates, he sent ray particles through various experiments changing the position of the
charged plates. By changing the plates, he discovered he could also change the point at
which the particles would hit a florescent screen at the end of the tube. He decided that
the ray was made up of particles with a negative charge. He named these particles
electrons. This discovery also showed that atoms had a substructure. Thomson made a
model showing his idea for the atom. His model, called the raisin dough model had
positive and negative particles imbedded into a positive atom.
Other particles in the atoms nucleus were discovered after the electron thanks to
Albert Einstein. In his theory of relativity, proposed in 1905, Einstein developed the
equation E=mc2, with E representing energy, m representing mass and c representing the
velocity of light. This meant that because the nucleus made up such a large percentage of
a atoms mass, the atoms energy came from the nucleus.
New Zealand scientist Ernest Rutherford began experimenting with radioactivity in
the early 1900s. Rutherford put a radioactive substance in front of two opposite charged
plates. Some of the rays coming out of the radioactive sample bent up towards the
positively charged plate. He named this negative ray beta. Another ray moved
towards the negative plate. He named this positive ray alpha. A third ray named gamma
made no movement towards either of the plates and was determined to have no charge.
Radioactivity helped future scientists to discover the true model of an atom.
Rutherford continued his study with another experiment. In this one, he shot a beam of
alpha rays through a piece of gold. Although most passed straight through, some
scattered. Rutherford knew that these alpha rays had bounced off a positively charged
particle. With this data, Rutherford made a new model, the planetary model, in which
electrons were orbiting around a positive nucleus.