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Atomic Theory and Structure of
the Atom
A. Democritus
• first to suggest the
existence of atoms
• believed atoms were small
indivisible particles
• Atom: smallest particle of
an element that retains all
properties of the element.
(460 B.C. – 370 B.C)
B. Dalton
• Atomic theory composed of 4
– Elements are composed of atoms
– Atoms of the same element are
identical, each element is unique
– Compounds are composed of atoms
of more than 1 element, atoms
combine in fixed ratios to form
– Chemical rxn involves separation,
combination, or rearrangement of
C. Electron
1. Symbol: e−
– Charge −1
**after Dalton, there was a big hiatus in the theory
of the atom. Everyone accepted Daltons theory and
went on to study radiation for a while, and this is
when the electron was discovered
D. J.J. Thomson
• An atom is thought of as a uniform positive
sphere with electrons evenly distributed
through out (like plum pudding or a chocolate
chip cookie)
Name of theory
Plum Pudding
Cathode Ray Tube
Evacuated tube
E. Millikan
Millikan Oil Drop
• Found the
mass of an
electron to be
9.10×10−28 g
F. Rutherford
• Conducted gold foil experiment
What did Rutherford expect?
here for
Gold Foil Experiment Animation and
What did Rutherford actually see?
F. Rutherford
• Concluded all positive charge and mass is
concentrated in small region called nucleus
and the electrons are outside the nucleus
• Proposed atom is mostly empty space
Name of theory
Nuclear Model
– p+
– Charge: +1
– Mass:1.673×10−24 g
G. Bohr
• Expanded on the nuclear atom
• Electrons travel in circular orbits around
the nucleus
Name of theory
Planetary Model
H. Schrödinger
• model allowed the electron to occupy
three-dimensional space like an electron
Name of theory
Quantum Mechanical
Model or Electron
Cloud Model
I. Chadwick
The Neutron
Symbol: n0
Charge: none
Mass:1.675×10−24 g
**The neutron was found by comparing the mass of hydrogen to
helium, since He increased by 1proton, the mass should be double,
but it was four times, leading to a subatomic particle that had mass
but no charge.
Fun Facts
• The volume of a hydrogen nucleus is a trillion
times smaller than the volume of a hydrogen
atom, yet the nucleus contains most of the
• If the nucleus (proton) of a hydrogen atom
were as large as the width of a human thumb,
the electron would be on the average about
one kilometer away in a great expanse of
empty space.