The History of the Atom
Democritus
(c.460-c.370 BCE)
• Democritus was a
philosopher in ancient
Greece.
• He thought that all matter
was made of tiny particles
that could not be divided.
• The Greek word atomos
means ‘indivisible’.
John Dalton
(1766-1844)
• Dalton was an English
scientist.
• He developed modern
atomic theory.
• His model of the atom is
sometimes called the
‘billiard-ball model’.
Dalton’s Billiard-Ball Model
• All matter is made of tiny
particles called atoms.
• Atoms of the same element
are identical.
• Atoms can combine to form
compounds.
• Chemical reactions change
the grouping of atoms, but
not the atoms themselves.
Sir J. J. Thomson
(1856-1940)
• Thomson was a British
physicist.
• He did experiments on
cathode rays and discovered
the electron.
• In 1906, he was awarded a
Nobel prize for his discovery.
• His model of the atom is
called the ‘plum-pudding
model’.
Thomson’s Plum-Pudding Model
• Dalton realised that negatively
charged electrons could come
from an atom.
• He proposed the ‘plumpudding’ model of the atom,
suggesting that atoms consist
of negatively-charged
electrons in a ‘sea’ of positive
charge.
Ernest Rutherford
(1871-1937)
• Rutherford was a chemist from
Nelson, New Zealand.
• Based on the results of his goldfoil experiment, he proposed
that most of the mass of an
atom is concentrated in a
central nucleus.
• He won the Nobel Prize in
Chemistry in 1908 for his work.
Rutherford’s Gold-Foil
Experiment
• In his famous gold-foil experiment, Rutherford
fired alpha particles at a thin sheet of gold foil.
• He found that some particles were deflected
through large angles, and some even bounced
back.
Rutherford’s Nuclear Model
Rutherford concluded that:
• most of the mass in an atom
must be in a very small,
positively-charged nucleus
in the centre of the atom
• electrons spin around this
central nucleus
• there was a basic unit of
positive charge in the
nucleus, called the proton.
• Niels Bohr (1885-1962)
realised that the electrons
could only occupy fixed
orbits around the nucleus.
• Louis de Broglie (1892-1987) proposed that
electrons can be regarded as waves, resulting in
an ‘electron cloud’ around the nucleus.
• Sir James Chadwick (1891-1974) discovered the
neutron, a nuclear particle with similar mass to a
proton but no electrical charge.
References
Atom Image: http://www.turbosquid.com/FullPreview/Index.cfm/ID/197928
Democritus: http://smccd.net/accounts/goth/MainPages/Chron/Democritus.jpeg
Dalton: http://en.wikipedia.org/wiki/John_Dalton
Thomson: http://en.wikipedia.org/wiki/J_J_Thomson
Plum-Pudding Atom: https://reich-chemistry.wikispaces.com/file/view/348pxPlum_pudding_atom_svg.png
Ernest Rutherford: http://en.wikipedia.org/wiki/Earnest_Rutherford
http://nobelprize.org/nobel_prizes/chemistry/laureates/1908/rutherford-bio.html
Gold-Foil Experiment:
http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter02/Text_I
mages/FG02_05.JPG
Nuclear Atom:
http://upload.wikimedia.org/wikipedia/commons/thumb/e/e1/Stylised_Lithium_Atom.svg/1
80px-Stylised_Lithium_Atom.svg.png
Electron Cloud: http://www.csmate.colostate.edu/cltw/cohortpages/viney_old1/atom.jpg