Sciencemadness Discussion Board

orbital , electron cloud

aeacfm - 11-11-2011 at 05:54

i discussed the electron cloud with my friend
i said electron cloud represent all orbitals in the atom , he said no electron cloud is due to electron moving every where in space ......who is right ???

Chemistry Alchemist - 11-11-2011 at 07:16

Im pretty sure the electron cloud is the space where the electrons are known to exist... the darker the cloud, the more likly the electron will be... im just guessing but im pretty sure thats what it is... may need to get a expert to answer tho

Neil - 11-11-2011 at 08:03

The electron cloud represents the most statistically likely areas to find the electrons at any given times.

See

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

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

And last but most;

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

aeacfm - 11-11-2011 at 08:58

Quote: Originally posted by Neil  
The electron cloud represents the most statistically likely areas to find the electrons at any given times.

l



what i think when talking about the whole electrons of the atom the above sentence is the case , but when talking about specific electron so you are talking about the orbital

kavu - 11-11-2011 at 09:43

Electron cloud is merely a simplification of the orbital model. Orbitals are the probabilistic representations for solutions of hydogen atoms Scrödingers equation. There is no good mathematical model for interactions of two electrons. For this reason orbitals for multielectron systems have to be approximated using the solutions for hydrogen.

Orbitals are visualized as blobs in space, a neat way to interpret the mathematical model. This has nothing to do with what they really are. One way to draw the blobs is to figure out where in space the electron is found with 90% chance. Then draw a surface accordingly and call it an orbital. In theory you could find an electron say a meter from the nucleus, this is just highly improbable.

Given that this quantum mechanical interpretation of the nature is correct, each electron of a multielectron atom has a certain wavelike nature which satisfies the qm-conditions. These different wave particles are superimposed in space. Using the blob-visualization this would mean that all the orbitals are on top of each other, leading to a great big blob. This blob would be considered to be the "electron could".

aeacfm - 11-11-2011 at 10:36

Quote: Originally posted by kavu  
These different wave particles are superimposed in space. Using the blob-visualization this would mean that all the orbitals are on top of each other, leading to a great big blob. This blob would be considered to be the "electron could".


this exactly what i think , thank you

watson.fawkes - 11-11-2011 at 14:56

Quote: Originally posted by kavu  
There is no good mathematical model for interactions of two electrons. For this reason orbitals for multielectron systems have to be approximated using the solutions for hydrogen.
This is false. You just put the electrostatic interaction between electrons into the Hamiltonian and solve the Schrodinger equation as before. This was first done for the helium atom and was the thing that really convinced people that the QM formalism was right, since semi-classical models predicted the hydrogen atom adequately.

aeacfm - 11-11-2011 at 21:37

Quote: Originally posted by watson.fawkes  
Quote: Originally posted by kavu  
There is no good mathematical model for interactions of two electrons. For this reason orbitals for multielectron systems have to be approximated using the solutions for hydrogen.
This is false. You just put the electrostatic interaction between electrons into the Hamiltonian and solve the Schrodinger equation as before. This was first done for the helium atom and was the thing that really convinced people that the QM formalism was right, since semi-classical models predicted the hydrogen atom adequately.


i had read the below quote in general chemistry ; whitten page 214


Quote:

The wave function for an atom simultaneously depends on (describes) all of the electrons in the atom. The Schrödinger equation is much more complicated for atoms with more than one electron than for a one-electron species such as a hydrogen atom, and an explicit solution to this equation is not possible even for helium, let alone for more complicated atoms. We must therefore rely on approximations to solutions of the many-electron Schrödinger equation. We shall use one of the most common and useful, called the orbital approximation.


that says all are approximations

[Edited on 12-11-2011 by aeacfm]

watson.fawkes - 12-11-2011 at 06:04

Quote: Originally posted by aeacfm  
that says all are approximations
The interaction model is exact; that's what's in the Hamiltonian. The solutions used for calculation are not analytically closed; that's what's approximated. So while there's a perfectly good exact model for the equation, there's not for the solutions. This is the generic case for most practical differential equations.

fledarmus - 12-11-2011 at 08:44

Quote: Originally posted by watson.fawkes  
Quote: Originally posted by aeacfm  
that says all are approximations
The interaction model is exact; that's what's in the Hamiltonian. The solutions used for calculation are not analytically closed; that's what's approximated. So while there's a perfectly good exact model for the equation, there's not for the solutions. This is the generic case for most practical differential equations.


In other words, it is much easier to determine the exact equations necessary to describe the electronic states than it is to solve them.

White Yeti - 12-11-2011 at 12:42

You can also think of electrons as standing waves around the nucleus, in which case, they would be everywhere at once;). The quantum world is strange, I prefer not to think of all the physics involved and stick to the idea that electrons are bits of matter floating around the nucleus that get shared and transferred between atoms, makes life so much easier.

arsphenamine - 18-11-2011 at 22:17

Quote: Originally posted by fledarmus  
In other words, it is much easier to determine the exact equations necessary to describe the electronic states than it is to solve them.
Yes. That doesn't mean you can't get usable results in good time when you run quantum chem calculations, because people do so all the time. The larger, more interesting compounds always take more time, though usually less time than an equivalent lab experiment.

But, it's arcane and very complicated. QM/MM software is a confluence of 100 years of physics and mathematics, a somewhat lesser period for chemistry, and 40 years of software engineering evolution.

Gimarc's method of "Qualitative Molecular Orbitals" is a very useful bag of concepts and worth a review.

Panache - 19-11-2011 at 07:50

Technically the cloud has no beginning or end, a point particularly relevant. If one chooses any point in space, anywhere in the universe, that point, which is dimensionless, can be assigned a probability of finding an electron there, from any atom in the universe. While this may sound completely pointless and absurd its a very elegant means with which to describe the concept of an electron, an orbital and matter in general. Consider the orbital as simply a probability distribution map, as in the probability of finding an electron there.
I found this generalized description insightful. It's however blindingly simplistic and helps little with the quantitative treatment necessary to aid in application of theory to real chemistry systems.
The beauty is because one is examining a point, the whole Pauli bit is uncircumstantial as an electron cannot possibly ever be dimensionless.