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The Structure of the Atom
Chemistry – Chapter 4
Early Theories of Matter
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Philosophers
◦ Democritus was first to propose Atomic
Theory:
 Matter composed of empty space through which
atoms move
 Atoms are indivisible
◦ Aristotle rejected Atomic Theory
 Respected for ideas on nature, physics, astronomy,
etc., so most ignored Democritus’ ideas
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John Dalton
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All matter composed of atoms
Atoms can not be divided
Different atoms combine to form compounds
Atoms are separated, combined, or rearranged in
chemical reactions
◦ Conducted convincing experiments

Atom – smallest particle of an element that
still retains the properties of the element
◦ Can move individual atoms around to form
shapes, patterns, and simple machines
 Nanotechnology
Subatomic Particles and the Nuclear
Atom

Electron discovery
◦ Sir William Crooks discovered the cathode
ray
 Led to invention of TV
 Cathode ray particles carry negative charge
◦ J.J. Thomson found that mass of charged
particle was much less than that of a H atom
 This meant Dalton was wrong and atoms are
divisible
 Identified the electron
 Plum pudding model (p. 94 fig 4-9)
◦ Robert Millikan determined charge of
electron
 Single electron carries charge of -1
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Nuclear atom
◦ Ernest Rutherford concluded plum pudding
model was incorrect
 Calculated atom consists of mostly empty space
through which electrons move
 Concluded there is a small, dense region in the
center that contains all positive charge and virtually
all mass (nucleus)
 Nuclear model (p. 95 fig 4-12)
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Discovery of protons and neutrons
◦ Rutherford concluded the nucleus contains
positively charged particles (protons)
 Protons carry a charge of +1
◦ James Chadwick showed nucleus also contains
a neutral particle in the nucleus (neutron)
 Mass nearly equal to proton
 Neutral charge
◦ Electrons are held within atom by attraction
to positively charged nucleus
◦ Number of protons equals number of
electrons
How Atoms Differ

Atomic number
◦ Defined as the number of protons in an atom
◦ Determines element’s position on periodic table
◦ Atomic number = proton # = electron #
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Isotopes - all atoms of an element have same
number of protons and electrons, but
number of neutrons differ (isotopes)
Mass number – sum of proton # and
neutron #
◦ Number of neutrons = mass # - atomic #
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Mass of individual atoms
◦ Protons and neutrons have approx. same mass
◦ Electrons are MUCH smaller
◦ B/c the masses are so small (must use
scientific notation, which is cumbersome),
chemists developed a standard for
measurement
 Carbon-12 atom
 Exactly 12 atomic mass units (amu)
 1 amu is 1/12 the mass of carbon-12 atom
◦ Atomic mass of an element is weighted
average mass of the isotopes of that element
Unstable Nuclei and Radioactive
Decay

Radioactivity
◦ Chemical reactions involve only an atom’s
electrons
 Nucleus remains unchanged
 Atom identity does not change
◦ Nuclear reactions involve change in atom’s
nucleus
 Atom of one element changes into atom of another
element
◦ Radioactivity – some substances spontaneously
emit radiation
◦ Radiation – rays and particles emitted by the
radioactive material
◦ Radioactive atoms emit radiation b/c their nuclei
are unstable
◦ Radioactive decay - unstable nuclei lose energy by
emitting radiation spontaneously

Types of radiation
◦ Experiment conducted by scientists in late 1800s
determined some radiation was deflected toward
positively charged plate, some toward negatively
charged plate, some not at all
◦ Alpha radiation – radiation deflected toward
negatively charged plate
 Alpha particles
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2 protons, 2 neutrons
+2 charge
Equivalent to He-4 nucleus
Represented by α
◦ Beta radiation – radiation deflected toward positively
charged plate
 Fast-moving electrons called beta particles
 Beta particle is 1 electron with -1 charge
 Represented by β
◦ Gamma rays – high-energy radiation that possess no
mass and no charge
 Represented by γ
 B/c massless, emission of gamma rays can not result in
formation of new atom
◦ Nuclear stability
 Primary factor is ratio of neutrons to protons
 Atoms w/ too few/many neutrons are unstable
 Few radioactive atoms in nature
◦ Nuclear equation – shows atomic #, mass #, and
particles involved
 Both mass # and atomic # are conserved