Download Atoms - SWThornton

History, Theory,
Structure
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How can we describe something we can’t
even see?
Compare to Black Box Experiment
Can’t see internal structure
Discovery by experimenting with how matter
behaves
Constantly changing and/or refining
definition
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Our current understanding of atomic
structure is based on:
Nucleus
Contains
Protons
(+) and
Neutrons
(N)
+
N
+
Electron
Shells (Not
exact
locations)
e-
Drawn as an
outer “wall” only
for reference;
Energy
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Aristotle: matter may
be continuously
divided without
changing its intrinsic
properties
Democritus:
matter
is composed of tiny,
indivisible particles;
atoms may not be
divided
“Ancient”
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Dalton: Atoms may
not be further divided.
Thomson: pudding
theory; electrons
Rutherford: gold foil
Bohr: quantum theory
Schrodinger: chargecloud
“Modern”
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1808
English
Two very important tenets
◦ Law of Conservation of Mass
◦ Law of Definite Proportions
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All matter is made up of atoms.
All atoms of an element are identical in size,
mass, and chemical properties.
Atoms of an element are different from atoms
of all other elements.
Atoms may neither be divided nor destroyed.
Atoms may be combined, separated, and
rearranged to form new compounds.
Atoms of different elements combine in
whole-number ratios to form compounds.
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1911
Rutherford’s Gold Foil Experiment
Actually conducted by Rutherford’s assistant
Hugely advanced for time period
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Bohr’s model related structure and position
of electrons to energy levels
Ground state
Excited state
Line-emission spectrum
Continuous spectrum
Only worked consistently for hydrogen
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Based on Bohr’s ideas with one main difference
Electrons are not traveling in fixed positions
Use of orbitals rather than orbits
Areas where electrons are most likely to be found
For example, an
electron will
never be expected
to be found in the
atom’s nucleus
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Protons have a positive charge.
◦ Mass of 1 unit
◦ Located in nucleus
◦ Identity of an atom
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Neutrons have no charge
◦ Mass of 1 unit
◦ Located in nucleus
◦ Help to stabilize nucleus
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Located outside of the nucleus
◦ Negative charge
◦ Mass is insignificant to mass of whole atom
◦ Responsible for chemical reactivity of an atom
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Though mass is tiny, equal but opposite
charge
◦ Can change position
◦ Relate to energy of an atom
Sub-Atomic Particles
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Positive
charge
Located in
nucleus
Determine
properties of
an atom
Unique to
each element
Protons
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No charge
(neutral)
Located in
nucleus
Thought to
provide
stability to
nucleus
Numbers
may vary
(isotopes)
Neutrons
•Negative charge
•Located in
energy levels
around nucleus
•Responsible for
chemical
reactivity of an
atom
Electrons
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All mass measurements are comparisons
based on standards
All atoms based on mass of carbon-12 atom
Weighted average of all known isotopes
based on measured frequency
Combination of protons and neutrons
12.011
C
6
Atomic Mass
•represents
combined
masses of
protons and
neutrons
Atomic Number
•Represents
number of
protons; in a
neutral
atom
represents
number of
electrons
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While all atoms of an element have the same
number of protons, numbers of neutrons may
differ
These atoms still retain the basic properties
of the element
Small differences in behavior
Atoms of the same element with differing
numbers of neutrons
Indicated by symbol with number to indicate
number of neutrons or mass
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Mendeleev (1834-1907); Russian; 1880’s
◦ listed approximately 73 known elements in columns
in order of increasing atomic mass
◦ left blank spaces for unknown elements
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Moseley (1887-1915); British; worked w/
Rutherford; 1913
◦ determined atomic number of atoms of known
elements
◦ arranged elements by atomic number
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Periods
◦ Horizontal
◦ 7
◦ Periodic Law
◦ Properties change as you go from left to right on
the Periodic Table.
 Atomic mass increases
 Atomic number increases
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Group or Family
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Vertical
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Similar physical and chemical properties
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Significance of Group Number
◦ For main group elements represents number of
valence electrons
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Approximately 80% of known elements
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Alkali: Group I
◦ Highly reactive
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Alkaline Earth: Group 2
◦ Highly reactive
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Transition, inner transition
Properties
◦ malleable, ductile, lustrous
◦ solids at room temp except Hg
◦ good conductors of heat and electricity
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Not lustrous
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Poor conductors of electricity and heat
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Halogens
◦ Group 7
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Noble gases
◦ Group 8
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Right side of Periodic Table
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Zig zag line
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Properties of both metals and nonmetals
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Used in computer chips and solar cells;
semiconductors
Periodic Table of the Elements
Source: Jefferson Labs
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Historical perspective
◦ Models of atoms based on behavior rather than a visual
analysis
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All atoms of an element have same basic
properties
◦ Isotopes
 Atoms of same element with differing # of neutrons
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Identity of an atom based on protons
Chemical reactivity of an atom based on
electrons
Periodic Table of the Elements
◦ Organization and Patterns