J. J. Thomson (1856 – 1940)
John Dalton
(1766–1844)
Ernest Rutherford (1871 – 1938)
UEQ: How does the structure of matter influence its physical and chemical
behavior?
 Atom
 Electron cloud
 Isotope
 Nucleus
 Proton
 Neutron
 Electron
 Mass number
 Atomic number
 Average atomic
mass
History of the Atom
Democritus (400 BC) proposed idea
that all matter is made up of
atoms
History of the Atom
Aristotle disagreed and said that
matter was uniform throughout
and not composed of smaller
particles.
History of the Atom
Dalton (1800’s) proveD atoms DiD
exist BUT he thought the atom was
a solid sphere
History of the Atom
Thomson (1904) proposed electrons
were dispersed throughout a
positively chargeD sphere (“plumpuDDing”)
History of the Atom
Rutherford (1911) disproved
thomson’s moDel by using the golD
foil method. This proved that there
was a dense, positive center
surrounded by electrons.
History of the Atom
Bohr (1913) hypothesized electron
travel around fixed orbits around
the nucleus
Chadwick concluded the nucleus was
made of protons and neutrons
History of the Atom
Electron Cloud model (current)
Electrons do not follow fixed orbit;
but, tend to be at certain areas
around nucleus at any given time
Structure of the atom
3 subatomic particles identify the element
1. protons
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Ernest Rutherford (1909)
(+) charge
Relative mass 1.007 276– or ~1 amu
Mass of element
Made of 3 quarks
In nucleus, held by nuclear forces
Identifies the element
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2. Neutron (N)
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James Chadwick (1932)
No charge; neutral
Relative mass = 1.008 665 – or ~1 amu
In nucleus and held by nuclear forces
isotopes- atoms of same element with different
#’s of neutrons
 Contributes to mass of atom
 Made of 3 quarks
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3. Electron (e-)
J.J. Thomson (1897)
( – ) charge
Relative mass 0.000 5486
In orbitals, within the electron cloud
Makes volume of atom (100,000 times larger
than diameter of nucleus)
Valence electrons - # in last energy level
determines the chemical activity
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Atomic Number
 Number of protons
 Can also tell you number of electrons
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Atomic Mass
 Number of protons and neutrons of an atom
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Average Atomic Mass
 Weighted average mass of the mixture of an
element’s isotopes
Example: We have a box of 2 types of marbles
(100 total). If 25% of these marbles have a
mass of 2 g and 75% have a mass of 3 g, then
what is the average mass of the marbles?
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Mathematical Formulas
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# of neutrons = mass # - atomic #
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Mass Number = neutrons + protons
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Atomic # = mass # - # of neutrons
Nuclear Symbol
superscript
subscript
Hyphen notation
 Element (name or symbol) hyphen mass #
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Isotope
Elements of same atomic number with
different number of neutrons
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Common Isotopes and Uses
Hydrogen 1,2 Tracers for metabolic studies
Oxygen 16 Metabolic studies and determining
temperature of ancient seas
Carbon 12 Metabolic studies and dating recent
archeological artifacts
Phosphorus 31 Metabolic and ecological tracers,
studies of nucleotides and nucleic acids
Sulfur 32, 33, 34 Labeling Proteins
Cobalt 59 Cancer therapy, source of gamma rays