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Atomic Model History
Early Ideas
Greeks thought that there were 4 elements
that made up ALL matter:
– air, earth, fire, and water
Democritus – first to suggest that
all matter was made up of some
tiny, fundamental particle.
– Called this particle “ATOMOS” –
meaning “indivisible”
John Dalton
• Very first atomic
theory (what atoms
were made of and
how they behaved)
• Viewed atoms as
tiny, solid spheres
“Billiard Ball” Model
Dalton’s Atomic Theory
1. Atoms are tiny, indivisible particles
2. All atoms of one element are the same.
3. Atoms of different elements are different.
4. Atoms of different elements combine to form
compounds.
J.J. Thomson
“Chocolate Chip
Cookie” Model
(or “Plum Pudding”)
• Cathode-ray tube
experiments
• Discovered electrons
(particles with a negative
charge)
• Was the first to show that there are
smaller pieces that make up an atom.
Ernest Rutherford
Gold Foil Experiment –
positively charged alpha particles were
passed through a thin sheet of gold foil.
– Most particles continued
through on a straight path
– Discovery: the
atom is made
of mostly
empty space
Ernest Rutherford
Gold Foil Experiment (cont.)
– Some particles were deflected at
large angles
– Discovery: atoms have a very
dense positive (+) center,
called the nucleus
“Peach” Model
(Positive nucleus and mostly
empty space)
Niels Bohr
• Electrons move around the
nucleus in specific layers, or
shells.
• Chemical properties of each element are
determined mainly by the number of
electrons in the outermost shell
James Chadwick
• Discovered neutrons
– Particles with no charge
found in the nucleus
Modern Atomic Model
Schrödinger & Heisenberg
• Electrons move around the nucleus in a cloud
• It is impossible to know the exact location of
an electron. (There are 3-D areas that they have a
probability of being found in – more on this later… )
“Quantum Mechanical”
Model