Download Lecture 10 - Atoms & Spectra

Why Learn about Atomic Structure?
• Knowing the structure
of atoms tells us about
their
– chemical properties
– light-emitting
properties
– light-absorbing
properties
An example of absorption spectra
from many different types of stars.
• From this information
we can learn about
galaxies, stars, planets,
asteroids, based on the
light they emit or
reflect.
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Atomic Structure
• An atom is composed of a dense core
called a nucleus and surrounding this
nucleus one or more negatively
charged electrons.
• The nucleus is composed of positively
charged protons and neutral neutrons.
• The electron is also 1,800 times
lighter than a proton. Protons and
neutrons however weigh about the
same.
• The electric force of attraction
between the positive protons and
negative electrons keeps the electrons
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bound to the nucleus.
Atomic Structure
• An atom is mostly empty space because the electron
moves around the nucleus at such a great distance. If
the proton were 1 cm wide a hydrogen atom would be
larger than a football field!
• A chemical element is determined based on how
many protons the nucleus contains (Hydrogen has 1,
Carbon has 6, Oxygen has 8 protons).
• When two atoms with the same number of protons
have different numbers of neutrons the two atoms are
isotopes of one another (Carbon has 6 protons but can
have 6, 7, or 8 neutrons).
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Ions
• An atom normally has the same
number of electrons as protons.
With the same number of
positive and negative charges,
an atom normally has no net
charge.
• If an atom loses or gains one or
more electrons it has become
ionized. With too few electrons,
the atom has a net positive
charge, too many electrons and
it has a net negative charge.
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Quantum Structure of an Atom
• An electron does not orbit a nucleus like a
planet orbits the Sun.
• For a given atom there are only a select few
orbits that an electron can occupy.
• This means that the orbits are quantized.
• Electrons may shift between these quantum
levels with either the emission or absorption
of a photon of electromagnetic radiation.
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Quantum Structure of an Atom
• If an electron absorbs a
photon of light it can shift
to a higher quantum level
• If an electron emits a
photon of light it can shift
to a lower quantum level
• The energy and
wavelength of the photon
in both cases depends on
the energy difference
between the two quantum
levels.
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Spectra and Atomic Structure
• Each type of atom has a
unique set of wavelengths
of light that it can absorb
and emit
– Hydrogen emits red light
at 656 nm, blue light at
486 nm and other lines
– Hydrogen will also absorb
only red light at 656 nm,
blue light at 486 and a few
other lines
• We use this to identify the
atoms present by studying
the spectrum of an object
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Emission or Absorption Spectra
• The brightness of an
emission line or the
darkness of an
absorption line
indicates how many
atoms are absorbing
or emitting that
color.
• The number of atoms
absorbing or
emitting depends on
the number present
and on the
temperature of the
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gas.
Types of Spectra
• Continuous Spectra from hot, dense or
solid objects.
• Emission Line Spectra
- from hot, tenuous
(thin) gas.
• Absorption Line
Spectra - from cold,
tenuous gas through
which light from a hot,
dense object passes.
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Types of Spectra
• Emission and
absorption
spectra are
sometimes
shown as graphs
of intensity
versus frequency.
• Several
molecules have
been identified in
this spectra of a
comet.
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Doppler Shift
• If a source of light is
moving towards or
away from an observer
its spectral lines are
shifted based on the
speed and direction
• The faster the object is
moving the greater the
shift
• Shifts to longer
wavelengths means
object is moving away.
Shorter , coming
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closer.
Doppler Shift
• Radar guns used by
police to catch speeders
use Doppler shift to
determine speed.
• Astronomers refer to an
increase in wavelength
(object moving away) as
a redshift. A decrease in
wavelength (object
moving closer) is called a
blueshift.
• This technique is also
used to search for planets
around other stars.
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The Motion of Stars
This star is moving away from
Earth. The light from this star will
appear redshifted.
• Stars are rarely ever
moving exactly towards or
away from you.
• The doppler shift only
gives one component of
the motion.
• Sometimes photographs of
stars taken many years
apart can reveal the other
components of stellar
motion.
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