Courtesy of astronomyonline.org
Courtesy of hubpages.com
Year
Scientist(s)
Discovery
4th Century B.C.
Democritus
First person to formulate the atomic structure
theory
1704
Isaac Newton
Proposed a mechanical universe with small solid
masses in motion
1803
John Dalton
Proposed an atomic theory with atoms having
the same structure for every element
1832
Michael Faraday
1897
J.J. Thomson
Studied the effect of electricity on solutions,
coined term "electrolysis" as a splitting of
molecules with electricity,
Famously described the atom in the plum
pudding model with the raisins being electrons
and the pudding being positively charged cloud
1909
R.A. Millikan
Oil drop experiment determined the charge
(e=1.602 x 10-19 coulomb) and the mass (m =
9.11 x 10-28 gram) of an electron.
1911
Ernest Rutherford
He shot alpha particles at a thin gold foil. He
established that the nucleus was: very dense,
very small and positively charged. He also
assumed that the electrons were located outside
the nucleus.
1922
Niels Bohr
Proved similarities in the periodic table. His
atomic model had electrons circling the atom in
orbital shells
1932
James Chadwick
Using alpha particles discovered a neutral
atomic particle with a mass close to a proton. He
discovered the neutron.
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J.J. Thomson's Plum Pudding model
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Raisins are the negatively charged
particles(electrons)
Electrons float in a pudding of positive
charge (equal to negative charge) which
makes neutral atom
Ernest Rutherford

Gold foil experiment
 Alpha particles fired at gold. Some are
deflected which proves a small dense
nucleus in the center.
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Niels Bohr


Bohr model, shows electrons orbiting in
specific energy shells (each shell
containing a set electron limit) around
nucleus
Robert Millikan

Found the mass of electron to be 1/1836 amu
J.J. Thomson’s model (top), Rutherford (bottom) Courtesy of en.wikipedia.org

Positively charged subatomic particles in a atom

In a neutral atom # of protons= # of electrons
 The positive charge and negative (electrons) charge should cancel each other out

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Located in nucleus of atom along with neutrons
Mass of 1amu
Equal to the atomic number
Protons define which atom is which element
If # of protons change then it becomes a different
element
Courtesy of nasa.gov
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Subatomic particles with no charge
The mass of neutron nearly equals that of a
proton (1amu)
Located in the nucleus of the atom
Protons and neutrons together in the nucleus
equals the atom’s atomic mass

The nucleus is the most dense part of an atom for
this reason
Courtesy of about-chemistry.blogspot.com


Negatively charged subatomic particle
Surrounding nucleus is electron cloud



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This is where electrons lie in specific orbital shells
Mass of 1/1836 amu, almost zero
The electron cloud accounts for very little of
the atom’s mass
Ions are formed by loosing electrons (positive
ion) and by gaining electrons (negative ions)
Courtesy of greenenergybuild.com
Courtesy of renesas.com
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The nucleus is in the center of the atom
Contains protons and neutrons
Almost all the mass is contained in the nucleus
Nucleus is very small compared to the actual atom
 If the atom were the size of a stadium the nucleus
would be a marble
Nucleus is very dense
 A nucleus the size of a pea would weigh 250 tons
Courtesy of www.safetyoffice.uwaterloo.ca

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Atomic number is the number of protons in a
atom
Protons = electrons in a neutral atom
The periodic table is ordered by atomic
numbers
Mass=protons +
neutrons
Courtesy of msu.edu

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Isotopes are atoms of the same element with
different neutrons and atomic mass
For example: Carbon is mostly found with 6
protons and 6 neutrons (Carbon-12)
Carbon can also have 7 neutrons (C-13) or 8
neutrons (C-14) although this happens more rarely
To calculate avg. atomic mass of element
(isotope # 1 total mass)x(abundance in nature-percentage)
 Repeat for each isotope and then add answers together to
get avg. atomic mass

Courtesy of radiation-scott.org
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Bowles, Richard. "Atomic Structure." Richard Bowles Website. N.p., 29 Sept.
2010. Web. 29 Sept. 2010. <http://richardbowles.tripod.com/chemistry/
structure/structure.htm>.
Buescher, Lee. "Atomic Structure Timeline." Lee Buescher's Science Lab.
Watertown High School, 2010. Web. 25 Sept. 2010.
<http://atomictimeline.net/index.php>.
Carpi, Anthony. "Atomic Structure." The Natural Science Pages. N.p., 1999. Web.
25 Sept. 2010. <http://web.jjay.cuny.edu/~acarpi/NSC/3-atoms.htm>.
Wilbraham, Antony C., Dennis D. Staley, and Michael S. Matta. "Atomic Structure."
Chemistry. Expanded 4th ed. Menlo Park: Addison-Wesley , 1997.
82-106. Print.
Courtesy
mrstrong.wikis.msad52.org
Courtesy of
mrjarretwalker.blogspot.com