History of
Atomic Theory
Timeline
400-350BC
1803
1897
1911
1912
1913
1924 -1928
1932
1932
Greek Philosophers’ Views
Modern Atomic Theory
Electron Discovered
Protons & Nucleus Discovered
Electron’s Charge Measured
Bohr Model of the Atom
Quantum Model of the Atom
Neutron Discovered
Lithium Atom Split
Greek Philosophers’ Views
Democritus 400 BC
Matter is made of
small particles called
atoms. These
particles can not be
divided any further.
Correct, but
rejected
Aristotle 350 BC
Matter is made of air,
water, fire and earth. It
can always be divided
into smaller pieces.
There is no base unit.
Incorrect, but
accepted
1803
Dalton’s Modern Atomic Theory
1.
2.
3.
4.
5.
Matter is made up of atoms that are
indivisible and indestructible.
All atoms of an element are identical.
Atoms of different elements have different
weights and different chemical properties.
Atoms of different elements combine in
simple whole numbers to form compounds.
Atoms cannot be created or destroyed.
Latter proven wrong
1897 – Electron Discovered
JJ Thomson deflected a stream of
rays away from a negatively
charged plate and toward a
positively charged plate. He
concluded that these rays were
negatively charged electrons.
He created the Plum Pudding
atomic model. He thought
electrons were evenly mixed
among the positive charges of an
atom like plums are mixed in plum
pudding.
1911
Proton & Nucleus Discovered
Ernest Rutherford, Ernest
Marsden and Hans Geiger
proved Thomson’s Plum
Pudding model wrong.
He shot positively charged
alpha particles at a thin
gold foil. Most of the particles passed through,
but some were deflected sideways and
backwards. Rutherford concluded that most of
the atom is empty space and that protons are
positively charged particles and are located in a
dense nucleus at the center of the atom.
1912
Electron’s Charge Measured
Robert Millikan
Measured the
Negative charge
of an electron
using his famous oil
drop experiment.
His value is within
½% of today’s
accepted valued.
1913
Bohr Model of the Atom
Niels Bohr developed
a model of the atom
that showed electrons
orbiting the nucleus in
distinct energy levels,
much the way the
planets orbit the sun.
This model explained
the atomic emission
spectrum for hydrogen but it did not explain
the emission spectra for other elements.
1924 – 1928
Quantum Model of the Atom
Louis DeBroglie, Werner
Heisenberg and Erwin
Schroedinger worked
separately to propose
that electrons had
wave-like characteristics
and moved around the
nucleus within complex
regions known as orbitals.
This theory is still accurate nearly 100 years later.
1932
Neutron is Discovered
Scientists had
long known that
the mass of an
atom was about
double the mass
of its protons. However, it was James Chadwick
that finally discovered that a neutral particle with
about the same mass as a proton existed in the
nucleus. This particle was called a neutron.
This disproved Dalton’s 2nd Law that all atoms of
the same element are identical. Isotopes of an
element are atoms with the same number of
protons but different numbers of neutrons.
1932
Atoms are Split
John Cockcroft and Ernest Walton bombarded
lithium atoms with protons and split them into
two helium.
This disproved Dalton’s 5th Law that atoms
could not be divided, created or destroyed.