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Atoms:
Development of the
Atomic Theory
S.MORRIS 2006
Who are these men?
In this lesson, we’ll learn about the
men whose quests for knowledge
about the fundamental nature of the
universe helped define our views.
Democritus

460 BC - Greek philosopher proposes
the existence of the atom

He pounded materials until he made
them into smaller and smaller parts

He called them atoma which is Greek
for “indivisible”.
Democritus

His Theory:
All atoms:
 Are small hard particles

Are made of a single material formed into
different shapes and sizes

Are always moving, and they form
different materials by joining together
John Dalton

1803 - British chemist; elements
combine in specific proportions to form
compounds
Solid Sphere Model or
Bowling Ball Model
Proposed by John Dalton
John Dalton

His Theory:

All substances are made of atoms that
cannot be created, divided, or destroyed.

Atoms join with other atoms to make new
substances.

Atoms of the same element are exactly
alike, and
atoms of different elements are different in
mass and size.

J.J. Thomson

1897 - English chemist and physicist;
discovered 1st subatomic particles
Plum Pudding Model
or Raisin Bun Model
Proposed by J.J. Thomson
J.J. Thomson

His Theory:

Atoms contain negatively charged
particles called electrons and positively
charged matter.

Created a model to describe the atom as a
sphere filled with positive matter with
negative particles mixed in

Referred to it as the plum pudding model
Ernest Rutherford

1912 - New Zealand physicist
discovered the nucleus
Nuclear Model
Proposed by Ernest
Rutherford
Ernest Rutherford

His Theory:

gold foil
Small, dense, positively
charged particle present in
nucleus called a proton
gold foil
helium nuclei

Electrons travel around the
nucleus, but their exact places
cannot be described.
helium
nuclei
Niels Bohr

1913 - Danish physicist; discovered
energy levels
Bohr Model or Planetary
Model
Proposed by Niels Bohr
Niels Bohr

His Theory:

Electrons travel around the nucleus in
definite paths and fixed distances.

Electrons can jump from one level to a
path in another level.
Erwin Shrodinger

1924 - Austrian physicist; developed
the electron cloud model
Electron Cloud Model
Proposed by Erwin
Schrodinger
Erwin Shrodinger

His Theory:

The exact path of electrons cannot be
predicted.

The region referred to as the electron
cloud, is an area where electrons can
likely be found.
James Chadwick

1932 - English physicist; discovered
neutrons

His Theory:



Neutrons have no electrical charge.
Neutrons have a mass nearly equal to the
mass of a proton.
Unit of measurement for subatomic
particles is the atomic mass unit (amu).
The Atom’s Family Album!
Levels of Organization
Parts of Matter
smallest
largest
Atoms Family Album Cover
Now if you a real ‘G’, as in GENIUS…… What
you know about this??? Which element is it?
Modern Theory of the Atom

Atoms are composed of three main
subatomic particles: the electron, proton, and
neutron.

Most of the mass of the atom is concentrated
in the nucleus of the atom.
Modern Theory of the Atom

The protons and neutrons are located within
the nucleus, while the electrons exist
outside of the nucleus.

In stable atoms, the number of protons is
equal to the number of electrons.
Modern Theory of the Atom

The type of atom is determined by the
number of protons it has.

The number of protons in an atom is equal to
the atomic number.
Modern Theory of the Atom

The sum of the number of protons and
neutrons in a particular atom is called the
atomic mass.

Valence electrons are the outermost
electrons.