Atomic Theory
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Transcript of Atomic Theory
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Atomic Theory
Chapter 4
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Atomic Theory¨ Science is based off of observations.¨ A Scientific Law is a summary of what is
seen in observations. ¨ A Scientific Theory is an explanation of
why these observations are occurring.¨ Both laws and theories are tested by using
them to predict what would happen in certain situations.
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Atomic Theory¨ -Explains why all matter acts as it does
because it is composed of tiny particles called atoms, the basic building blocks of all matter
¨ First theorized by Democritus ~400 BC in ancient Greece.
¨ He thought that matter could NOT be divided infinitely. You had to reach a smallest possible piece. He named this piece an atom, which meant indivisible or can’t be cut.
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More Democritus¨ He and his followers, atomists, said atoms
were small hard particles all made of the same material, but different sizes and shapes.
¨ They were also always moving and capable of joining together.
¨ Since this was ancient Greece, He and the atomists had no way of ever proving his theories and few people believed it.
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Law of Conservation of Mass¨ A chemical reaction does not change the
mass of the matter present.¨ The mass of the reactants is equal to the
mass of the products.¨ This is true even if a different phase of
matter is created.
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Law of definite proportions¨ 1790’s¨ All pure compounds have exactly the same
proportions by mass of elements regardless of size
¨ Water is always 2 hydrogen atoms to every 1 oxygen atom
¨ By mass that is 2 (g) H : 16 (g) O
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Dalton’s Model (1803)¨ people started to accept the idea of atoms
because of his experiments¨ He worked with gases and found that they
acted as though they were made of solid microscopic particles
¨ all elements are made of atoms (indivisible and indestructible)
¨ atoms of the same element are exactly alike¨ atoms of different elements are different¨ compounds are formed by joining two or more
elements
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John Dalton
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Thomson’s Cathode Ray
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Thomson’s Model (1897)
¨ found negative particles could come from neutral elements
¨ atom is made of smaller things (+ & -), and is divisible
¨ successfully separated negative particles (electrons) but could not separate the positive particle (protons)
¨ “plum pudding model” negative particles floating in a positively charged gel like material
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Plum Pudding Model- Thomson
Positive Gel
NegativeParticles
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Sir J. J. Thomson The sir meanshe was knighted
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Rutherford’s Model (1911) ¨ fired protons at a sheet of gold foil most
went through unaffected, some bounced away
¨ there is a small dense area of positive particles at the center of the atom- the nucleus
¨ electrons are scattered near the outside of the atom with mostly empty space between the nucleus and the electrons
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Gold Foil Experiment
Radioactive source
Gold foil
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Rutherford’s Model
nucleus(small dense positive area)
electrons
Empty Space
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Ernest Rutherford
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Bohr Model (1913) ¨ electrons move in definite orbits around the
nucleus ¨ these orbits or energy levels are located at
certain distances from the nucleus
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Bohr’s Model
nucleus
Electrons
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Neils Bohr
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Wave Model (present day) ¨ based on complex math equations¨ orbits are more complex than originally
thought¨ de Broglie stated that electrons
(particles) have wave properties, and he viewed these as standing waves, like those produce when a guitar string is plucked (classical physics.)
¨ Schrodinger assumed that the electron in Hydrogen behaves as a standing wave.
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Wave Model (continued)¨ When Schrodinger’s equation is
analyzed, many solutions are found.
¨ Each solution represents an atomic orbital.¨ An atomic orbital is the most probable
location for finding an electron.
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What is an orbital?¨ It is not a Bohr orbit (not moving in
a circular path.) ¨ How is the electron moving?¨ We don’t know! ¨ There is a fundamental limitation
to just how precisely we can know both the position and momentum of a particle at a given time
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This is kind of how we assume an electron travels
e-
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Heisenberg Uncertainty Principle¨ The more accurately we know the
particle’s position, the less accurately we can know it momentum and vice versa.
¨ We can’t know the exact motion of the electron around the nucleus.
¨ The area that an electron orbits is called an “electron cloud”
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Louis de Broglie Erwin Schrodinger
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Werner Heisenberg