Chapter 18
Atomic Models
Presented by April Senger
Viewing the Atom
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The atom is difficult to view because it is
smaller than a wave of visible light
The number of atoms in one baseball
would be like counting the number of ping
pong balls it would take to fill a sphere the
size of the Earth
The Scanning Tunneling Microscope allows
us to “see” atoms indirectly by dragging a
tiny needle across the surface and using a
computer to create a topographical image
This is called nanotechnology
Presented by April Senger
Models
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Physical models are for small, large,
slowly occurring, or quick observations
The atom is best represented by a
conceptual model
Conceptual models look at the whole
system
The economy, population growth, the
spread of disease, and team sports are
also best represented with conceptual
models
Presented by April Senger
Concept Check
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A basketball coach describes a
playing strategy to her team by way
of sketches on a game card. Do the
illustrations represent a physical or
conceptual model?
Conceptual
Presented by April Senger
Spectroscopes & Atomic Spectrum
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The electrons are represented in the
atomic model by shells or energy levels
Every atom has a distinct number or shells
and electrons and can absorb energy
differently
When the energy absorbed by an atom is
released, it gives off a specific atomic
spectrum
We can use a spectroscope through
diffraction to split the colors of the
rainbow out like a prism
Each atom has its own rainbow fingerprint
Presented by April Senger
Concept Check
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How might you tell what elements are in a
star?
A spectroscope can analyze their spectral
patterns and identify their fingerprints
We tried this with our own sun in the
1800’s and found Hydrogen (already
identified) and another we named Helios
after the Greek word for sun-common
name Helium
Presented by April Senger
Historical Outline
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Democritus was a Greek philosopher that
theorized that everything was made of
atoms but there was no evidence.
John Dalton said that atoms were
indestructible spheres of solid matter (the
billiard ball model). They can be joined
together to form molecules. He had
evidence.
In 1913, Niels Bohr decided that the atom
was like a solar system where the planets
were the electrons and the sun was the
nucleus.
Presented by April Senger
More History
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Each electron has its own specific energy and
that determines what path it can take around the
nucleus. These are called energy levels.
Electrons behave like waves and particles so
Bohr’s theory didn’t work
We developed the Modern Atomic Theory that
says that electrons are not like planets. Electrons
are more like vibrating strings.
Electrons are hard to find…Think of a spinning fan
and each individual blade. It is impossible to
measure both an electron’s speed/direction and
location
Presented by April Senger
Bohr
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The light released is
called a photon and a
quantum is the
smallest unit of light
The quantum
hypothesis states that
a steady stream of
light is actually a
continuous stream of
tiny packets of light
Bohr used the amount
of light released to
determine the energy
levels of an atom
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He stated that the
lowest level is n = 1,
then 2, 3 and so on
This model started to
look like the solar
system so it is thus
named the Planetary
Model
The light emitted by
atoms was equal to
the energy it would
take to jump between
levels
Presented by April Senger
Shells
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Shells are a region of space about the
atomic nucleus within which electrons may
reside
Each shell holds a different number of
electrons
The shells hold the following number
electrons from level 1 up 2, 8, 8, 18, 18, &
32
The electrons that are in the outermost
level are the ones that determine an
atoms ability to chemically react
We call these electrons Valance Electrons
Presented by April Senger
Valance Electrons & Their Patterns
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Group 1 has one valance
Group 2 has two
Group 13 has 3
Group 14 has 4
Group 15 has 5
Group 16 has 6
Group 17 has 7
Group 18 has 8
The rest of the elements vary and some have
more than one possible number of valance
electrons
Presented by April Senger
Shell Model
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We know that the shell model is not
the most accurate
It is the easiest for us to draw
without the aid of a computer
Mostly, we use concentric circles
such as those found on page 318
The outer most electrons will be
useful in drawing Lewis Dot
Structures
Presented by April Senger