Chapter 6 Electronic Structure of Atoms continued

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Chapter 6Electronic Structure of
Atoms(continued)
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The Bohr Model
Johann Balmer observed that these lines for H2
occurred at very specific wavelengths, which
could be described by the equation
Where ? wavelength in meters RH
Rydberg constant 1.097 x 10-7 m-1 n1 and n2
are positive integers and n2 gt n1
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Bohr Model of the atom
Looking at the atom as a central nucleus
surrounded by electrons came from Rutherfords
experiment. Bohr assumed that the electrons
moved in orbits, but why did the electrostatic
forces not pull the electron in and the atom
continuously lose energy as electromagnetic
radiation? Plancks idea must apply - QUANTA of
light (in other words, very specific packets of
energy in the form of light were emitted.
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Bohr model

• Based on three postulates
• - Only orbits of certain radii (certain
  energies) exist for electrons
• Specific energy corresponds to allowed energy
  state
• Can only emit or absorb light when going between
  these allowed energy states (orbits)

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Bohr model

• Using these postulates combined with physics
  governing particle with charge of electron and
  given kinetic energy a potential energy due to
  interaction with nucleus, he calculated that the
  energies of the electron when it was in these
  specific orbits was

Where n is called a quantum number n1 is
ground state ngt1 is an excited state Ninfinity
corresponds to no interaction with nucleus. E
0 But is this consistent with Balmers
observations??
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Bohr model

• Using Bohrs model

Using Balmers observations
But ?? c or ? c/v, so
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Bohr Model
But RHhc -2.18 x 10-18 In other
words,Bohrs model correctly predicts the lines
that Balmer saw in the visible emission spectrum
of hydrogen. For atoms with more than one
electron, however, it does not explain
spectra. However, we still use the terms quantum
number, ground state and excited state when we
begin to use quantum mechanics, and we continue
to use the ideas of (1) Only discrete energy
levels allowed (described by quantum numbers) (2)
Require energy to get from one level to another