Development of the
Atomic Theory
Chapter 4.1
Pages 113 – 118
Democritus


Greek philosopher
born 450 B.C.
The universe is
made up of
invisible units
called atoms.
John Dalton


English school
teacher
Around 1800 A.D.
proposed the
“Atomic Theory”
which was widely
accepted.
Atomic Theory
Every element is made of tiny, unique
particles called atoms that cannot be
subdivided.
 Atoms of the same element are
exactly alike.
 Atoms of different elements can join
to form molecules or compounds.

J.J. Thomson



Cathode Ray tube
experiment (see page 115)
Showed that atoms have
negative particles.
Discovered the electron.
Earnest Rutherford



Gold Foil Experiment
There is a positive mass in
the center of each atom.
Discovered the nucleus
Models of the Atom




John Dalton – Billiard Ball Model
J.J. Thomson - Plum Pudding Model
Niels Bohr – Planetary Model
Louis de Broglie & Erwin Schrodinger –
electron cloud model
John Dalton’s Model


Solid Sphere Model or
Billiard Ball Model
An atom is a tiny
indivisible particle
J.J. Thomson’s Model - 1904


Also known as the
“Plum Pudding
Model” or “Chocolate
Chip Cookie Model”
Atom is a dense
positively charge
particle with negative
electrons embedded all
throughout.
Niels Bohr’s Model - 1922

Also known as the
“Planetary Model”.

Electrons move in
set paths around
the nucleus much
like the planets
orbit the sun.
Louis de Broglie &
Erwin Schrodinger





Electron Cloud Model
An atom has a positive nucleus in the
center.
Electrons are located in an area called a
“cloud” around the nucleus.
These clouds have different shapes.
You cannot find an electron’s exact location.
Correcting Dalton’s
Atomic Theory

Not all atoms of the same element are
exactly alike.


Existence of isotopes
You can subdivide an atom into parts.

protons, electrons, neutrons