Atoms:
Development of the
Atomic Theory
1
Democritus

460 BC - Greek philosopher proposes
the existence of the atom

His theory:
all atoms are small hard particles
 made of a single material formed into
different shapes and sizes
 always moving, and that they form
different materials by joining together

2
John Dalton


1803 - British chemist; elements
combined in specific proportions to form
compounds.
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 (elements)
3
Edward Frankland

1852 - English chemist developed the
valence theory

His theory:
 every
atom has a fixed number of bonds
(chemical links) that it can form
 for the atom to be stable, all of these
bonds must be used.
4
J.J. Thomson

1897 - English chemist and physicist
discovered 1st subatomic particles

His theory:
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

5
Ernest Rutherford

1912 - New Zealand physicist
discovered the nucleus

His theory:
 small,
dense, positively charged particle
present in nucleus called a proton
 electrons travel around the nucleus, but
their exact places cannot be described
6
Niels Bohr

1913 - Danish physicist discovered
energy levels

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
7
Erwin Shrodinger

1924 - Austrian physicist developed
the electron cloud model

His theory:
electrons exact path cannot be predicted
 regions, referred to as the electron cloud,
are areas where electrons can likely be
found.

8
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)
9
Modern Theory of the Atom




Atoms are composed of three main
subatomic particles: the electron, the
proton, and the neutron.
Most of the mass of the atom is concentrated
in the nucleus 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.
10
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

The sum of the number of protons and
neutrons in a particular atom is called the
atomic mass
Valence electrons are the outermost
electrons and are where bonding takes
place

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