Atoms and Elements
30 September 2004
Physics
Chemistry
Astronomy
Geology
Biology
Topics
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Discovery of what an atom is composed
Models of atoms
Bohr atom (1913)
 Quantum mechanics (1930’s)
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Elements and atoms
Electrons and shells/orbits
Valence electrons
Synopsis
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st
(1
lecture)
Principles of physics, astronomy, chemistry, geology,
and biology
Dominant theories and laws that underlie how the
natural world operates
Theories and laws are relevant in every day life
Underlying themes across all disciplines (e.g., emergent
properties, models, scientific method)
Cardinal features of scientific investigations (e.g.,
scientific method, data visualization, models, data
collection), through a combination of computer
simulations and hands-on experiments in lab
Models in the Sciences
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Conceptual models
based on observations
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Example: Dynamics of family
interactions
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Example: Structure of the
atom (protons, neutrons and
electrons
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Models are an abstraction and
are meant to be challenged
Protons and
Neutron
Electrons in
orbits
Scientific Method
Observations
Rejection/Acceptance of
Hypothesis
Proposal/Hypothesis
Testing Hypothesis
Crystals: Structure and Color
Topics
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Discovery of what an atom is composed
Models of atoms
Bohr atom (1913)
 Quantum mechanics (1930’s)
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Elements and atoms
Electrons and shells/orbits
Valence electrons
Compounds to Subatomic Particles:
A Hierarchy
Compound
Element
Atom
Subatomic Particle
Thompson’s Discovery of the
Electron
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Idea that there may be some smaller
components that comprise atoms
Hypothesis: Atoms consist of subcomponents,
one of which is negatively charged and very
small in mass
Experiment to test hypothesis
Thompson’s Experiment
Rutherford’s Discovery of a Nucleus
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Idea that there may be some smaller
components that comprise atoms, including
negatively and positively charges components
Hypothesis: Atoms consist of multiple
subcomponents, some negative and some
positive
Experiment to test hypothesis
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Alpha particles (+), gold foil, “bullets”, and
“tracks”
Rutherford’s Experiment:
Observations, Hypothesis and
Conclusion
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Observations
Almost all alpha’s passed through foil unaffected
 Very small number of alpha’s deflected @ small
angle
 1/1000 deflected at large angle (struck “head on”)
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Hypothesis: Atom consists of a positive
subcomponent that is very small in volume in
comparison to atom itself
Accept the hypothesis
Atomic Structure: Bohr Atom
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Observations (1913)
Heat hydrogen gas and light is emitted as a discrete
wavelength (not continuous spectrum)
 Other gases behave the same in producing discrete
wavelength, but each gas is unique in wavelengths
emitted
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Bohr Atom
A Model of Quantum Mechanics
Theory of Quantum Mechanics
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Observations
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Behavior of electrons in heated hydrogen gas consistent with
Bohr Model (orbits, etc.)
Behavior of other heavier gases not explained by Bohr Model
Investigators (Heisenberg, Shrodinger): wave-particle
duality of light
Key: integrated mechanisms of waves and particles,
focusing on “fuzzy electron clouds”/waves
Hypothesis: new model of atomic structure and
function: quantum mechanics theory of the atom
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Incorporation of know facts
Prediction of new properties yet to be discovered
Models in the Sciences
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Conceptual models
based on observations

Example: Dynamics of family
interactions

Example: Structure of the
atom (protons, neutrons and
electrons
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Models are an abstraction and
are meant to be challenged
Protons and
Neutron
Electrons in
orbits
Elements
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Patterns
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92 naturally occurring elements (e.g., hydrogen, gold, helium)
Total of 113
25 of 92 are essential to life (e.g., what are they?)
Key points
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any element is the same in its chemical structure and physical
properties (~stable over time)
All elements have origin in either the big bang (hydrogen and
helium) or the subsequent evolution of the universe
Elements
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Compound
Elements combine in very precise ways that are recurrent and
predictable
Sodium + Chlorine = Sodium Chloride
Na
+
Cl
=
NaCl
metal
+ gas
=
solid
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Key points
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Atoms of sodium (Na) and chlorine (Cl) remain atoms of
each
Emergent property: “emergence” of new properties in a
compound not be explained by the summation of the two
elements (hierarchy theory)
Particles
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Atoms are composed of particles (subatomic particles)
Most stable particles
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Neutrons
Protons
Electrons
Other less stable particles (quarks, neutrinos, etc.)
 Relationship among the more stable particles
Neutron
Proton
Electron
Charge
neutral
positivenegative
Mass
2x10-24 g
2x10-24 g
5x10-28 g
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Atomic Structure
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Atoms of the same element (e.g., hydrogen, helium,
gold) have the same number of subatomic particles and
by convention we abbreviate as follows:
2
Helium
# of protons
He
4
Abbreviation of element
Atomic mass (g/mole)
1
Hydrogen
H
1
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Mass =
Protons + Neutrons
Electrons
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Background of quantum mechanics
Energy “barons” of the atom (motion)
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Energy = ability to do work
Potential energy = energy stored due to position or location
Charge is negative (-) and particle is always in motion
Capturing an atom and its orbiting electron
Key to Electron Structure
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Count the number of electrons (and compare
with abbreviation of element)
Electrons are negative in charge and in constant
motion
Electrons are in orbits around the nucleus
Chemists sometimes refer to orbits as “shells”
Electrons
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Example of Sulfur (1632S)
(16 electrons in 3 orbits)
Example of Electrons in Shells
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As electrons move between shells, they change
potential energy
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Hot summer day, bright sun and car top
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Light absorption by pigments and electrons “jump” to
higher shell (potential energy); give off energy when they
drop back (kinetic energy)
Banana, orange juice or bagel this AM?
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Excited electron “captured” by chlorophyll in leaf and
shuttled to a sugar molecule in its excited state (potential
energy) until you release the energy via
digestion/respiration, allowing the electron to “drop back”
to a lower level (kinetic energy)
Some keys to Electron Structure
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Electrons reside in shells as a function of
quantum mechanics (1-4 orbits per shell)
Never more than two electrons per orbit (Pauli’s
Exclusion Principle)
Distribution of electrons is key to understanding
why elements and atoms behave the way they do
Outermost electrons are called valence electrons
and they have a very special significance in
chemistry
Question
According to the Rutherford model of the atom, the
volume of any atom is largely _______.
A.
protons and neutrons
B.
electrons
C.
empty space
D.
covertly sequenced nuons surrounded by
proton
Question
According to the Bohr model of the atom, an electron
gains or looses energy only by ______.
A.jumping from one atom to another
B. speeding up or slowing down in its orbit
C. jumping from one orbit to another
D.being removed from the atom
Question
According to the Rutherford model of the atom, the
volume of any atom is largely _______.
A.
protons and neutrons
B.
electrons
C.
empty space
D.
covertly sequenced nuons surrounded by
proton
Question
According to the Rutherford model of the atom, the
volume of any atom is largely _______.
A.
protons and neutrons
B.
electrons
C.
empty space
D.
covertly sequenced nuons surrounded by
proton
Discussion Question
As you scan the night sky, you see multiple objects, and
you question whether these objects are similar to or
different from Earth.
Suppose the person next to you says that she has an
instrument that can identify the occurrence of specific
atoms (e.g., sodium, hydrogen, sulfur) based on the
energy patterns/signatures of electrons (PE and KE) in
atoms on that planet. Could she be right?
Discussion Question
Models are one of the key investigative tools in the
scientific method. Models of our understanding of the
atom are a classic example of the role that models play
in the sciences.
List five features of models – not the specifics of any
given model, but rather how models are constructed,
how they serve to help promote our understanding of
natural systems, and their fallibility.
Discussion Question
You are in the atomic world, having won a free
all day ride on an electron nested within a
carbon atom. Your peers are jealous (hang with
me on this).
You are seated on an electron, duly belted down
with a safety harness and off you go.
Twenty-four hours later you return to life at
GMU. List the cardinal features of your “ride”.