Chapter 5.2
Remember where we stopped:
After the acceptance of the
Rutherford/nuclear model of the atom, the
two leading questions in the
chemistry/physics community were:
What are the electrons doing out there, and
why don’t they crash into the nucleus?
Why does each element emit a unique light
spectrum?
In 1913, Niels Bohr, a Danish physicist who
had studied with both Thomson and
Rutherford, devised an atomic model that
answered both questions:
Each element had a unique line spectrum
because each element had a unique electron
configuration (arrangement of the electrons
outside of the nucleus).
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The Bohr Model
says that electrons
are in discreet,
definite Principle
Energy Levels
outside the nucleus.
You will sometimes
here these referred
to as “shells”.
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Remember potential energy?
It’s stored possible energy that something
could have because of its position.
A ball held out a 1st floor window has less
potential energy than a ball held out of a
4th floor window. If dropped, the ball from
the 4th floor will hit with more energy than
the ball from the 1st floor.
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Remember conservation of energy?
Energy cannot be created or destroyed, but
it can be converted into other forms.
As the ball falls, PE but KE
After the bounce, KE and PE
Electrons in lower
energy levels (closer
to the nucleus) have
lower potential energy
than electrons in
higher energy levels,
just like a ball held
out of a 1st floor
window has lower PE
than a ball held out of
a 4th floor window.
nucleus
Planck’s Quantum Theory, Spectroscopy, and
the Bohr Model:
here we go…
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When an atom
absorbs a quantum
of energy, the
electrons may
jump up a level, or
several levels –
gaining PE. This is
the excited state.
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The PE they
gained (and are
now “losing”) has
to go somewhere…
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ground state.
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This is not stable,
and the electrons
quickly fall back
to the lower
levels, called the
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When the electrons fall back to lower levels,
they release a quantum of energy in the form
of light. The farther it falls, the more light
energy it emits (“larger” quanta).
Because each element has a unique
arrangement of electrons, each gives off a
unique light spectrum signature.
http://www.learnerstv.com/animation/animation.php?ani=124&cat=
chemistry
Atoms do not absorb every quanta of energy
that happens by; they will only absorb photons
that “fit” the PE difference between their
PEL.
The Bohr Model integrated the Rutherford
model of the atom and Plank’s quantum theory,
explained how atoms could absorb and emit
energy, why each element had a
characteristic light “signature”, and why the
electrons didn’t just crash into the nucleus.
There have been modifications to the Bohr
Model, but generally speaking, it forms the
basis of the modern atomic model.