Orbital versus Orbit
Date: September 2006
Can you please tell me the difference between orbitals and orbits?
I would like to know whether orbits actually exist, and how should I
try and visualize orbitals in different energy levels?
I often find that if I am confused to what the differences are
between two things, that looking up the definitions of the words in
question helps tremendously.
Taken from Dictionary.com, the definition of orbital is "a wave
function describing the state of a single electron in an atom
(atomic orbital) or in a molecule (molecular orbital)." and also
"the wave function of an electron in an atom or molecule, indicating
the electron's probable location." The definition of orbit is
"Physics. (in Bohr theory) the path traced by an electron revolving
around the nucleus of an atom.
To put things in simplest terms, an orbit is the path an electron
takes around an atom and an orbital is a shape resulting in the
combination of all probable orbits. In another example, let us take
the Earth orbiting the Sun. Each year the Earth completely revolves
once around the Sun. The specific path that you could draw out is
the orbit. But each year, the Earth takes a slightly different
path--whether it is slightly high or lower, or closer or further
away than last year. You can physically compare the two different
orbits that the Earth took. Now, let us say that you recorded the
Earth's orbit every year for a million years and then you combined
all of these orbits into one massive shape. You would result in
something that looked pretty much like a big doughnut. This
doughnut would be the orbital that Earth is confined to, much the
same an electron's orbit is to the nucleus. I believe that the
shape of an orbital is defined as where the electron is predicted to
be 99.99% of the time (or some very high probability). So that
resulting shape is the orbital.
Strictly speaking "electron orbits" refer to the Bohr Model of the
atom. This is a way of visualizing the fact that electrons in an
atom have a particular energy and are limited to a small set of
possible energies; the electron energies are quantized.
"Electron orbitals", on the other hand, refer to the Schrodinger
Model of the atom. In this model, mathematical equations are used to
describe the likelihood of an electron having a particular energy
being in a particular location in an atom.
Since both of these concepts are models of the atom, strictly
speaking, they are conceptualization, ways of imagining or
describing an atom. Whether the electron truly exist in a circular
orbit or in a mathematical orbital is irrelevant. What matters is
that, given the strict parameters of the model, the model is able to
predict certain chemical and physical properties of the atom.
Thus the orbit model of an electron is sufficient to predict the
light emission and energy absorption of the hydrogen atom. The
Schrodinger model, with its fuller description of electron energies
and probable locations, is able to do more than that.
Please be very careful in the visualization of both models. The
moment you visualize a model, you are translating the strict
parameters of the model into a visual metaphor and this can lead to
many misconceptions. While you do have to find a way to internalize
the model, try to do this without adding to the model ideas that are
not within the parameters of that model.
Greg (Roberto Gregorius)
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Update: June 2012