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Electron Velocity
Name: Unknown
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993
Question:
How fast do electrons move (under typical circumstances in a
typical atom)?
Replies:
We tend to think of atoms in electrons in an atom as like planets
rotating around the sun. While this view is attractive, the basis of
semiweeklies of the first theories of atom structure by Bohr, is a
simplification. Really, the electron should be considered as smeared out over
a large volume surrounding the atom. In this sense, the electron does not
move inside the atom.
Unknown
Well, that is all strictly true, but still it is possible to
ascribe approximate velocities to electrons in bound states. This is done all
the time to ascribe whether relativistic effects are important in calculating
these bound states; for example, relativistic contraction of the inner core of
electrons is the explanation typically used to explain the unique properties
of transition metals, and relativistic corrections to calculations are
currently a frontier area in theoretical atomic and chemical physics. A sort
of gross model for "speeds" of electrons in bound states can be obtained from
the Bohr model. This model predicts that the electron associated with a
hydrogen nucleus would be moving at 2.42 x 10^8 cm /sec, which you may wish to
compare with the speed of light: c = 3.00 x 10^10 cm/sec. So the velocity of
an electron in the first Bohr orbit (ground state) is a tiny fraction of the
speed of light, which is why non relativistic forms of quantum theory work
quite well for hydrogen. However, start increasing Z (the charge on the
nucleus) and the Bohr velocity for the inner electrons starts to get
huge...and experimentally, there is a considerable contraction of the spatial
extent of these electrons relative to H. So, although velocities are not
strictly defined for electrons whipping around a nucleus, an approximate model
(the Bohr model) does give one a sense of when one might need to start
formulating relativistic corrections to quantum mechanics.
Topper
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Update: June 2012
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