The Atomic Model
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The Atomic Model
Atomic Theory Timeline
The atomic model has changed over time.
For over two centuries, scientists have created different models of the atom.
As scientists have learned more and more about atoms, the atomic model has changed.
But First, Democritus!
Democritus was a Greek philosopher (470- B.C.) who is the father of modern atomic thought.
He proposed that matter could NOT be divided into smaller pieces forever.
He claimed that matter was made of small, hard particles that he called “atomos”
John Dalton - 1808
John Dalton created the very first atomic theory.
Dalton viewed atoms as tiny, solid balls.
Dalton was an English school teacher who performed many experiments on atoms.
His atomic theory had 4 statements…
J.J. Thomson (1897)
J.J. Thomson discovered electrons.
He also proposed the existence of a (+) particle…
His atomic model was known as the “raisin bun model”…
He was the first scientist to show that the atom was made of even smaller things.
Thomson’s Model
Atoms are made mostly out of (+) charged material, like dough in a bun.
The (-) charged electrons are found inside the (+) dough.
Rutherford’s Experiment
Radioactive material emits beam of (+) alpha particles
Gold foil Screen
Most particles went right through!
Strangely, some particles are deflected
Rutherford’s Experiment
Gold Nucleus
a a a a
Most particles went through the gold. The atom is mostly empty space.
Niels Bohr (1913)
Niels Bohr improved on Rutherford’s model.
Every atom has a specific number of electron shells.
He proposed that electrons move around the nucleus in specific layers, or shells.
James Chadwick (1932)
Chadwick discovered neutrons.
He called these particles neutrons.
Working with Rutherford, he discovered particles with no charge.
Neutrons are also found in the nucleus.
The Current Atomic Model
Protons
Neutrons
Electrons
Atomic Theory Timeline
Dalton Thomson Rutherford Bohr Chadwick Modern