Shapes of Orbitals = Probability spaces??



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Topic: Science > Physics
User: "Davis"
Date: 16 Aug 2005 04:11:05 AM
Object: Shapes of Orbitals = Probability spaces??
Couldn't the shapes of orbitals be considered probability
spaces of the electrons with the shapes being boundary surfaces
where there are more probability of the electrons being
found? Some don't agree and state that orbitals are energy
eigenstates and the shapes are left over from Bohr days.
What can you say about the following wikipedia entry where
shapes still exist even as surface boundary. You agree with
it? Anyone can edit the entry at the site.
http://en.wikipedia.org/wiki/Atomic_Orbital
The shapes of orbitals
Any discussion of the shapes of electron orbitals is necessarily
imprecise, because a given electron, regardless of which orbital it
occupies, can at any moment be found at any distance from the nucleus
and in any direction.
However, the electron is much more likely to be found in certain
regions of the atom than in others. Given this, a boundary surface can
be drawn so that the electron has a high probability to be found
anywhere within the surface, and all regions outside the surface have
low values. The precise placement of the surface is arbitrary, but any
reasonably compact determination must follow a pattern specified by the
behavior of =CF=882, the square of the wavefunction. This boundary surface
is what is meant when the "shape" of an orbital is mentioned.
Generally speaking, the number n determines the size and energy of the
orbital: as n increases, the size of the orbital increases.
Also in general terms, determines an orbital's shape, and its
orientation. However, since some orbitals are described by equations in
complex numbers, the shape sometimes depends on also. s-orbitals ()
are shaped like spheres. p-orbitals have the form of two ellipsoids
with a point of tangency at the nucleus. The three p-orbitals in each
shell are oriented at right angles to each other, as determined by
their respective values of .
Four of the five d-orbitals look similar, each with four pear-shaped
balls, each ball tangent to two others, and the centers of all four
lying in one plane, between a pair of axes. Three of these planes are
the xy-, xz-, and yz-planes, and the fourth has the centres on the x
and y axes. The fifth and final d-orbital consists of three regions of
high probability density: a torus with two pear-shaped regions placed
symmetrically on its z axis.
.

User: "Dr Photon"

Title: Re: Shapes of Orbitals = Probability spaces?? 16 Aug 2005 05:54:02 AM
Davis wrote:

Couldn't the shapes of orbitals be considered probability
spaces of the electrons

yes

with the shapes being boundary surfaces
where there are more probability of the electrons being
found?

yes, though one has to remember that the "boundary" is fuzzy, it just
sort of thins out. Where does the Earth's atmosphere really stop?

Some don't agree and state that orbitals are energy
eigenstates

eigenfunctions

and the shapes are left over from Bohr days.

no. Bohr had "orbits", where the electrons went around like little
planets. "Orbitals" are completely different, and are introduced by the
Schroedinger eqn. Take a look at
http://www.shef.ac.uk/chemistry/orbitron/
and click around. I can't imagine the Earth going around the sun like
any of those.

What can you say about the following wikipedia entry where
shapes still exist even as surface boundary. You agree with
it?

seems moderately ok.
br
.

User: "Bjoern Feuerbacher"

Title: Re: Shapes of Orbitals = Probability spaces?? 16 Aug 2005 10:43:45 AM
Davis wrote:

Couldn't the shapes of orbitals be considered probability
spaces of the electrons

Depends on what "probability spaces" is supposed to mean.

with the shapes being boundary surfaces
where there are more probability of the electrons being
found?

No. The shapes you talk about (those shown in the picture on the
webpage you cite below) are *not* actually the shapes of the orbitals,
but merely angular plots of the spherical harmonics, which determine
the *angular* part of the wavefunction.
Alternatively, you can draw a "boundary surface" as discussed in the
section "The shapes of the orbitals" on that webpage, that will give
the same pictures. But again, that are *not* the *actual* orbitals.

Some don't agree and state that orbitals are energy eigenstates

Well, they are. The pictures you are talking about are illustrations
of the orbitals, not the orbitals themselves.

and the shapes are left over from Bohr days.

Huh?????

What can you say about the following wikipedia entry where
shapes still exist even as surface boundary. You agree with
it? Anyone can edit the entry at the site.

http://en.wikipedia.org/wiki/Atomic_Orbital

See above.
Apparently you didn't understand what they say there.
[snip quote]
Bye,
Bjoern
.

User: "Y.Porat"

Title: Re: Shapes of Orbitals = Probability spaces?? 16 Aug 2005 06:11:00 AM
Have you ever heard about the idea of
'Chain of orbitals' ??
ATB
Y.Porat
------------------------------
.
User: "tj Frazir"

Title: Re: Shapes of Orbitals = Probability spaces?? 17 Aug 2005 12:10:46 AM
At the same time the electrons orbit can be shaped into stars box hex
square hex falf moon with ozzzies pic on it.
ALL the orbiting parts of the atom orbit the center of gravity at the
center of mass .
But that might combine that atoms gravity with a planets gravity. The
parts in orbit change to an eliptical orbit streched tward the planets
gravity. The center of mass is above the center of gravity.
The atom pushes its self.
F is identical to the distance from the center of gravity to the center
of mass.
.


User: ""

Title: Re: Shapes of Orbitals = Probability spaces?? 16 Aug 2005 09:13:39 PM
There is no relation between orbitals and Bohr's orbits: orbitals
derive from the principles of quantum mechanics and have a precise
physical meaning; Bohr's orbits are just a try to put some numbers
together and have only (limited) historical meaning. You say that same
people (who?) state that orbitals are energy eigenstates (?) and shapes
are non-sense (?); well, this is objectively and undoubtedly false.
First of all, orbital are functions . The only confusion in this matter
is whether one means wave functions (the eigenfunction of the energy
operator, called Hamiltonian) or their squared absolute values; the
latter is the commonest convention. Eigenvalues are numbers, not
functions.
The squared absolute value of a wave function (|ps|2 now on; I do not
want to use Greek letters now) has a precise, measurable, physical
meaning: the density of probability of finding the system in that point
of the space; thus you can calculate the probability of finding an
electron in a certain region by integrating the |ps|2 in that region.
This is a principle of quantum mechanics, just like Newton's laws in
classical machanics or Maxwell's laws in electromagnetism.
Anyhow there is a problem when we want to rapresent this |ps|2 since it
is a function of three indipendent variables (density of probability
for every point in three dimensional space), thus a four-dimensional
graph would be necessary. Since we leave in a three dimensional world
we cannot have a totally satisfactory graph of it, but we can rapresent
it in many different ways that can give a hint of the whole function.
This problem is similar to that of rapresenting temperatures of a
region on the earth on a two-dimensional map and in some cases the same
convenctions for graphs are used.
Here's a list of the most usual ways for rapresenting orbitals
(intended as |ps|2):
- isosurfaces: surfaces formed by points sharing the same value of the
function (like isotherms on the temperature map mentioned above). Since
this surfaces tipically contain each other, only one is rapresented;
which surface is arbitrary, but the surface containing the 90% of
probability (that's to say the integral of |ps|2 inside that surface is
0.9) is often chosen. This is the commonest rapresantation.
-shaded filling. The color of a point varies as the value of the
functions (the same solution is often used in a temperature map). These
graphs are beautiful, but not very clear.
- Angular probability. The graph is a surface; the distance of a point
from the center of the coordinates is equal to the integral of |ps|2 in
that direction from the centre to infinity; this is the density of
probability of finding the electron in that particular direction, no
matter how far from the nucleus. These graphs look similar to those of
isosurfaces, but they have a different meaning.
- Radial probability. These are two-dimensional graphs that show the
value of |ps|2 in function of the distance from the nucleus, integrated
around angles. This is the probability of finding the electron on a
spherical surface of that particular radius, no matter in which
direction.
|ps|2 is density of probability but the wave function (ps now on)
contains more information than |ps|2, that we might need. In fact ps
contains the full information about the state of the system (another
principle of quantun machanics). The rappresantation of ps is more
difficult than that of |ps|2 since it is a complex function and thus
would need a five-dimension graph to show it completely. The usual
choice is to put additional information about |ps|2 graphs in two
steps:
- the set of ps corrisponding to the same value of total angular
momentum (i.e. the orbitals that have the same letter: p, d, f) are
combined to make them real. This process is possible because all these
orbitals have the same energy, and the resulting orbitals are just as
good (as long as they are indipendent and normalized); anyhow the
original orbitals are also eigenfunctions of the projection of angular
momentum (they have a definite value of m), the resulting orbitals are
not: the electron still have a definite absolute value of m, but the
sign is lost, which means that electron can spin left or spin right
around the nucleus with equal probability. The orbital with m=0 is
already real and is left unchanged; this is simmetric around z axis and
for l>1 looks different from the other orbitals.
If we plot the probability of the original orbitals, we totally lose
the information about phase and the orbitals sharing the same absolute
value of m would have exactly the same graph.
-the memory of the sign is kept by plotting |ps|2 graph in different
colors corresponding to the sign in the original ps. Sign is very
important to understand interactions of bonding atoms and for many
other properties.
So, it does make sense to speak of the shapes of orbitals, and they
deeply affect properties of atoms. Orbitals are the distribution of
electron density in the space just like clouds are the distribution of
water in the sky (and they also have a shape).
Bye
.
User: "John Sefton"

Title: Re: Shapes of Orbitals = Probability spaces?? 17 Aug 2005 10:15:32 AM
wrote:

There is no relation between orbitals and Bohr's orbits: orbitals
derive from the principles of quantum mechanics and have a precise
physical meaning; Bohr's orbits are just a try to put some numbers
together and have only (limited) historical meaning. You say that same
people (who?) state that orbitals are energy eigenstates (?) and shapes
are non-sense (?); well, this is objectively and undoubtedly false.

First of all, orbital are functions . The only confusion in this matter
is whether one means wave functions (the eigenfunction of the energy
operator, called Hamiltonian) or their squared absolute values; the
latter is the commonest convention. Eigenvalues are numbers, not
functions.
The squared absolute value of a wave function (|ps|2 now on; I do not
want to use Greek letters now) has a precise, measurable, physical
meaning: the density of probability of finding the system in that point
of the space; thus you can calculate the probability of finding an
electron in a certain region by integrating the |ps|2 in that region.
This is a principle of quantum mechanics, just like Newton's laws in
classical machanics or Maxwell's laws in electromagnetism.

Anyhow there is a problem when we want to rapresent this |ps|2 since it
is a function of three indipendent variables (density of probability
for every point in three dimensional space), thus a four-dimensional
graph would be necessary. Since we leave in a three dimensional world
we cannot have a totally satisfactory graph of it, but we can rapresent
it in many different ways that can give a hint of the whole function.
This problem is similar to that of rapresenting temperatures of a
region on the earth on a two-dimensional map and in some cases the same
convenctions for graphs are used.
Here's a list of the most usual ways for rapresenting orbitals
(intended as |ps|2):
- isosurfaces: surfaces formed by points sharing the same value of the
function (like isotherms on the temperature map mentioned above). Since
this surfaces tipically contain each other, only one is rapresented;
which surface is arbitrary, but the surface containing the 90% of
probability (that's to say the integral of |ps|2 inside that surface is
0.9) is often chosen. This is the commonest rapresantation.
-shaded filling. The color of a point varies as the value of the
functions (the same solution is often used in a temperature map). These
graphs are beautiful, but not very clear.
- Angular probability. The graph is a surface; the distance of a point
from the center of the coordinates is equal to the integral of |ps|2 in
that direction from the centre to infinity; this is the density of
probability of finding the electron in that particular direction, no
matter how far from the nucleus. These graphs look similar to those of
isosurfaces, but they have a different meaning.
- Radial probability. These are two-dimensional graphs that show the
value of |ps|2 in function of the distance from the nucleus, integrated
around angles. This is the probability of finding the electron on a
spherical surface of that particular radius, no matter in which
direction.

|ps|2 is density of probability but the wave function (ps now on)
contains more information than |ps|2, that we might need. In fact ps
contains the full information about the state of the system (another
principle of quantun machanics). The rappresantation of ps is more
difficult than that of |ps|2 since it is a complex function and thus
would need a five-dimension graph to show it completely. The usual
choice is to put additional information about |ps|2 graphs in two
steps:
- the set of ps corrisponding to the same value of total angular
momentum (i.e. the orbitals that have the same letter: p, d, f) are
combined to make them real. This process is possible because all these
orbitals have the same energy, and the resulting orbitals are just as
good (as long as they are indipendent and normalized); anyhow the
original orbitals are also eigenfunctions of the projection of angular
momentum (they have a definite value of m), the resulting orbitals are
not: the electron still have a definite absolute value of m, but the
sign is lost, which means that electron can spin left or spin right
around the nucleus with equal probability. The orbital with m=0 is
already real and is left unchanged; this is simmetric around z axis and
for l>1 looks different from the other orbitals.
If we plot the probability of the original orbitals, we totally lose
the information about phase and the orbitals sharing the same absolute
value of m would have exactly the same graph.
-the memory of the sign is kept by plotting |ps|2 graph in different
colors corresponding to the sign in the original ps. Sign is very
important to understand interactions of bonding atoms and for many
other properties.

So, it does make sense to speak of the shapes of orbitals, and they
deeply affect properties of atoms. Orbitals are the distribution of
electron density in the space just like clouds are the distribution of
water in the sky (and they also have a shape).

Bye

http://www.petcom.com/%7Ejohn/Be.GIF
In this representation of four electrons
each of the four moving spheres represents
the center of a 'spiral of millions of particles
which extend from the center of the atom to
its edge'(and is constantly regenerating
through that center).
Bohr dreamed of fiery planets zipping
around a sun *and connected by filaments
to that sun*. One cannot put electrons
in different planes as Bohr tried to do
to make larger atoms because the filaments
would get tangled. You must keep all the
filaments moving the same rate, in the same
plane, all the time.
In the above animation, each electron cloud
can maintain its constant connection with
its proton, yet the four electrons rapidly
sweep out a sphere with two shells.
John
.



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