Electron Orbit Measurement
Date: February 2007
When measuring the orbit of a electron is the
measurement taken from the nucleus to the electron or is it the
circumference of the orbital? For instance: the orbit of electrons
in helium is about 0.03 nanometers. Is that measurement the
circumference of the orbit or the distance from the nucleus to the orbit?
That is the average distance between the electrons and the
nucleus. It is not a set, single position like a planetary orbit;
instead it is a distribution.
Department of Physics and Astronomy
University of Wyoming
First of all, let us make a clear distinction between "orbit" and
"orbital". Orbit (as in the Bohr planetary model of the atom)
relates (in a false imagery) the energy of an electron to a distance
from the nucleus. The Schrodinger Model of the atom corrected that
idea by stating that electrons have a set energy that may be
described by a set of variables which are only marginally related to
distance. In the Schrodinger model, there is a "most probable"
distance, often called a "shell" and we, unfortunately, imagine this
as a sphere around the nucleus. But truly, these are just ways for
us to visualize the mathematical description of an electron's energy
and are NOT truly physical manifestations of the electron's properties.
So what do scientists mean when they say a distance of so-and-so
nanometers? There are two relevant issues: (1) we can measure the
light coming out of atoms when electrons decay from high orbital
states to lower orbital states, and (2) we can measure the distances
of nuclei in bonded atoms (by measuring the bond energy) and relate
this to the overall dimensions of each of the bonded atoms. Note
that in both these cases, there is no mention of an actual
measurement of the distance of the electron to the nucleus, this
distance is inferred. In case 1, we only know the energy difference
between two orbital states - not the energy of each of the
individual orbitals. In case 2, we measure the *average* distance of
two nuclei, and infer on average the dimension of a single atom.
From this, is built the *erroneous* idea that electrons are at a
particular distance from the nucleus.
What the Schrodinger model emphasizes is that an electron has a
particular quantum of energy that is described by 4 different
variables - none of which state that the electron is at a particular
distance from the nucleus. The principal quantum number, n, (one of
the 4 variables) alludes to distance because it describes the idea
that as n increase, the most probable distance of the electron to
the nucleus also increases. However, this is not to be taken to mean
that the electron is at this particular distance (or shell, or
circumference). So what we really are saying is that electrons have
energy and this energy establishes distances of nuclei in bonded
atoms and most probable distances in individual atoms.
Greg (Roberto Gregorius)
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Update: June 2012