HISTORY OF THE ATOM
For centuries scientist and philosophers have speculated about the structure of the atom.
Lets examine a few of these people.
A. Democritus
 Greek philosopher who lived about 2000 years ago.
 He believed that matter could be broken down only so far before it reached a
point where it become indivisible
 He called this particle which was indivisible the “atomos”, which means
indivisible
 We now call it the ATOM
Dalton
 Developed his own atomic theory in the early 1800’s
Dalton’s Atomic Theory
1. All elements were composed of atoms. These atoms were indivisible
and indestructible.
2. Atoms of the same element were exactly alike.
3. Atoms of different elements were different.
4. Compounds are formed by the joining of atoms of 2 or more elements.
B. Thomson
 Believed that atoms were divisible and that there were particles which composed
them.
 Through studying cathode ray tubes (gases in tubes carrying an electric current),
he discovered negatively charged particles. Atoms were overall neutral, so atoms
must be comprised of charged particles.
 He called the negatively charged particle the “corpuscle”, which later became
known as the ELECTRON
 Atoms were neutral, so if the atom contains (-) particles, it must also contain (+)
particles to balance this charge. Thomson, however, never did discover this (+)
particle.
 Thomson developed a model. This model looked like plums scattered around in
pudding. He envisioned the atom looking like a bowl of pudding with the
charged particles scattered throughout the atom. He called his model the
“PLUM-PUDDING MODEL”.
C. Rutherford
 Rutherford performed experiments known as the “GOLD-FOIL
EXPERIMENTS”. These experiments involved firing a tiny stream of (+)
particles at a thin sheet of gold.
 Through his experiments he discovered that the atom had a center which was (+)
charged. This center of the atom became known as the NUCLEUS.
 He also said that the nucleus contained these positive particle which were also in
the atom. He called these particle PROTONS.
 Since the protons were all in the nucleus, the electrons were scattered around
outside the nucleus.
D. Bohr
 Bohr expanded on what Rutherford had predicted about the electrons being
scattered around the nucleus. He answered where they were scattered around.
 Bohr said electron resided outside the nucleus in ORBITALS.
 He said that electrons move around the nucleus in definite orbits much like the
planets around the sun.
E. Chadwick
 Discovered the neutron.