History of the Atom
Scientists and Their Contribution to
the Model of an Atom
Democritus
(460 BC – 370 BC)
• Proposed that all atoms are
small, hard, indivisible and
indestructible particles made
of a single material formed
into different shapes and
sizes.
• Aristotle did not support his
atomic theory
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Time+Line+Project
Aristotle
(350 B.C.)
• modified an earlier theory
that matter was made of
four “elements”: earth,
fire, water, air.
• Aristotle was wrong.
However, his theory
persisted for 2000 years!
John Dalton
(1766 – 1844)
 In 1803, proposed an Atomic
Theory which states:
o All matter is made of atoms.
o Atoms are indestructible –
can’t be divided.
o All atoms of one element are
exactly alike.
o Atoms join with other atoms
to make new substances.
 Calculated the atomic weights of
many various elements
Image taken from:
chemistry.about.com/.../JohnDalton.htm
J.J. Thomson
(1856 – 1940)
Image taken from:
www.wired.com/.../news/2008/04/d
ayintech_0430
 Proved that an atom can be divided into
smaller parts
 While experimenting with cathode-ray
tubes, discovered electrons
 Stated that the atom is neutral
 In 1897, proposed the Plum Pudding Model
which states that atoms mostly consist
of positively charged material with
negatively charged particles (electrons)
located throughout the positive
material
 Won a Nobel Prize
Chocolate chip cookie dough model
Ernest Rutherford
(1871 – 1937)
 In 1909, performed the Gold Foil
Experiment and suggested the following
characteristics of the atom:
o The protons are surrounded by
negatively charged electrons, but most
of the atom is actually empty space
o It consists of a nucleus, that contains
most of the mass of the atom
o This nucleus is made up of particles
called protons, which have a positive
charge


Did extensive work on radioactivity (alpha & beta particles,
gamma rays/waves) and was referred to as the “Father of
Nuclear Physics”
Was a student of J.J. Thomson
Image taken from:
http://www.scientificweb.com/en/Physics/Biographies/Er
nestRutherford.html
Niels Bohr
(1885 – 1962)
Image taken from:
commons.wikimedia.org/wiki/File:Ni
els_Bohr.jpg
 In 1913, proposed Bohr Modelsuggests electrons travel
around the nucleus of an atom
in orbits or definite paths.
 Additionally, electrons can
jump from a path in one level to
a path in another level
(depending on their energy)
 Won a Nobel Prize
 Worked with Ernest Rutherford
History of the Atom - Timeline
1766 – 1844
Antoine Lavoisier
Thomson
makesJ.J.
a substantial
the
number discovers
of contributions
electron
and
to the
field of
proposes the
Chemistry
Plum Pudding
Model 1871
in 1897
– 1937
Niels Bohr
proposes
the Bohr
Model in
1913
1887 – 1961
James
Chadwick
discovered
the neutron
in in 1932
1700s
1800s
1900s
460 – 370 BC
0
Democritus
proposes
the 1st atomic
theory
1743 – 1794
Erwin
John Dalton
Ernest Rutherford
Schrodinger
proposes performs
his
the Gold Foil
describes
1891 – 1974
atomic theory
Experiment
in
in 1909
the electron
1803
cloud in 1926
1885 – 1962
Click on picture for more information
1856 – 1940
Progression of the Atomic Model
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The structure of an atom, according to:
Electron Cloud
Democritus
James
Ernest
Erwin
Neils
Schrodinger
Chadwick
Rutherford
Bohr&
J.J.
Thomson
John Dalton