Count Valence Electrons for Element: Shell Model

[aaaa202]

How do you count the number of valence electrons for an element? I mean with the shell model it is pretty obvious but the electrons are actually in orbitals.

 

[Superhoben]

If I got it right the valence electrons are the electrons in the orbitals with the highest energy level.
You may have seen that the orbitals exists in somewhat shells with different energy levels. The amount of electrons in the shells with the highest energy level is the number of valance electrons.
The highest energy level can at most contain 8 valence electrons as after that the repulsion will cause the new electrons to orbit in a new energy level, when they become enough many on that energy level additional electrons may go to the previous energy level causing the elctrons on that level to exceed 8. But then we already have a higher energy level and those will no longer be apart of the valance electrons.
What I said isn't completely true and overall very simplyfied but I hope that it gives you an idea atleast.
You can read futher about it on wikipedia:
http://en.wikipedia.org/wiki/Electron_configuration
http://en.wikipedia.org/wiki/Electron_configurations_of_the_elements_(data_page )
I don't know much about this, I learned about it for one or two months ago but I will try my best if you got you wonder anything more.

 

[hjelmgart]

A valence electron, is an electron that can form a chemical bond with another atom/molecule, just like you know it from the shell model.

It is not any different, when you use orbitals, but there is of course some probability of finding the electron outside its orbital (in another orbital), which is described by quantum mechanics. The orbitals also overlap each other. But the way you count the amount of valence electrons, is still the same, because valence electrons will have a higher energy, and thus they will have the highest probability of being found in the outer most orbitals. It is also these orbitals, that are energetically unfavorable, and thus the atom will either need more energy or want to let some go, by accepting or donating an electron.

Of course once you start to look at "real-life" problems such as bulk compounds it becomes a bit more difficult, since the way two atoms form bonds can result in even more valence electrons, and thus give rise to conducting properties.

Also you have some metals, that can have valence electrons in more than one shell in the shell model, but I would recommend reading more into that stuff.

As a last note, I could mention the Pauli principle, which says that you cannot find two electrons at the same place. This also leads to the fact, that the probability of finding the electrons near each other is low. So the average locations of the electrons will be kind of like that in the shell model. This is also, why you can talk of a filled orbital, and you can use this to even explain the shape of the orbital depending on the amount of electrons.

Of course the mathemathics is not as "simple" as that, and it is after all, the math that proves it..

 

[Superhoben]

I thought that it was the repulsion that cause electrons to rarely be found close to each other, are you sure that it's because of the Pauli exclusion principle?

 

[hjelmgart]

You are of course correct, but repulsion is really due to the Pauli exclusion principle, as it is always about the quantum state of the electron.
But you can prove it by finding a wave function for the helium atom, where you will then see, that the electrons are most likely to be found at certain places a special distance from each other.

What I also meant, is that an electron in the outer orbits cannot enter a filled orbit due to the Pauli exclusion principle. There is a finite amount of electrons, that can occupy an orbital at a time, so the amount of electrons in the outermost orbit(s) is always the same. Which is why, you can use the shell model for counting.