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Atomic Theory
Chapter 4
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.
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.
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.
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.
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
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
John Dalton
Thomson’s Cathode Ray
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
Plum Pudding Model- Thomson
Positive Gel
Negative
Particles
Sir J. J. Thomson
The sir means
he was knighted
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
Gold Foil Experiment
Gold foil
Radioactive source
Rutherford’s Model
Empty Space
electrons
nucleus
(small dense
positive area)
Ernest Rutherford
Bohr Model (1913)
 electrons move in definite orbits around the
nucleus
 these orbits or energy levels are located at
certain distances from the nucleus
Bohr’s Model
nucleus
Electrons
Neils Bohr
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.
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.
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
This is kind of how we assume an
electron travels
e-
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”
Louis de Broglie Erwin Schrodinger
Werner Heisenberg