Transcript Chemistry You Need to Know

Section 4.7—Light & Matter
Electrons Absorbing Energy
Photon coming into atom collides with
electron. Photons are energy.
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Electrons Absorbing Energy
Photon coming into atom collides with
electron. Photons are energy.
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The electron is “excited” to a higher energy level with is
newly increased energy from absorbing the photon.
Excitation
The process of an electron absorbing a
photon of light (energy) and being
promoted to a higher energy level from its
“ground state”
And later…
The electron cannot remain in that excited state indefinitely
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And later…
The electron cannot remain in that excited state indefinitely
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Energy is released during relaxation
Relaxation
The process of an electron releasing a
photon of light (energy) and falling back
down to a lower energy level.
Energy of photon and levels jumped
The higher the energy of the photon, the
greater the electron jump!
A photon of UV light has more energy than
a photon of Infrared light
The UV photon would cause a higher energy
jump (jump up more levels) than the IR photon.
Total energy in = Total energy out
However much energy was absorbed must
be released again, but it can be released
in smaller packets
A high energy photon might be absorbed,
but two lower energy photons might be
released as the electron falls in a “stepwise” manner.
Photons must match energy changes
The energy of the photon must exactly
match the energy change of the electron.
If the photon is not an exact match, the
photon will pass through unabsorbed.
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Measuring light absorption
Not all
colors
come out
All
colors
of light
go in
Sample of hydrogen
Hydrogen Spectrum
The black bars are the colors that a hydrogen atom absorbs.
The other colors pass through the atom un-absorbed.
Absorption of Molecules
Because the structure of a molecule is
much more complicated than a single
atom, they absorb regions of light rather
than single wavelengths.
Absorption spectrum of water
Picture from http://www.lsbu.ac.uk/water/vibrat.html
Ways of producing light
Fluorescence: visible light is absorbed and
visible light is emitted at the same time—
the relaxation happens very quickly after
excitation
Phosphorescence: Visible light is
absorbed and then a while later is
emitted—relaxation occurs after a period
of time
Ways of producing light
 Incandescence: Energy is put in from heat and
given off as visible light
 Chemiluminescence: Energy released during a
chemical reaction is absorbed to cause
excitation. Relaxation produces visible light
 Biolouminescence: Chemiluminescence that
occurs in a biological organism.
 Triboluminescence: Physical pressure or torque
provides energy for excitation. Relaxation
produces visible light.
What did you learn about
glowing things?
Glow in the dark
Is based on
Electron
structure
Gives off
Changes in produce
Is a part of
Light
Which can
be determined using
Periodic
Table
Atomic
structure
Arranged to show
Is based on
Atomic
theory
Periodicity