ATOMIC STRUCTURE

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Courtesy of astronomyonline.org

History of Atomic StructureYear 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 Studied the effect of electricity on solutions, coined term "electrolysis" as a splitting of molecules with electricity,

1897  J.J. ThomsonFamously 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. 

Famous Experiments/Models

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

J.J. Thomson's Plum Pudding model 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.

Protons 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

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 elementCourtesy of nasa.gov

Neutrons 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

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Electrons Negatively charged subatomic particle Surrounding nucleus is electron cloud

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)

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Nucleus 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

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Elements/Atomic Numbers/Mass

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

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Isotopes 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

Sources 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.

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