Why Can't Classical Theory Explain Electron Affinity?

[hasan_researc]

My lecturer writes the following in his lecture notes:
“ Electron affinity: a QM property of atom to attract an electron, and increases the number
of electrons required to complete the outer electron shell decreases. “

I have known electron affinity as the desire, so to speak, of an atom to bring electrons in its shells. What I don’t understand is why classical theory can’t explain/predict this property of an atom !??

“ increases the number of electrons required to complete the outer electron shell decreases” !?? Is there a grammatical error in the sentence? Let’s assume that the corrected sentence would read “ the number of electrons required to complete the outer electron shell decreases”. My question is: why is there a limit to the number of electrons that can occupy the outer (or, in fact any) electron shell of an atom?

Any help would be greatly appreciated!

 

[20poundnote]

Hi, I'm a chemistry student. That statement could be better understood as "a property of an atom to attract an electron. Affinity increases as the number
of electrons required to complete the outer electron shell decreases“.

For example, Halogens (group 7/17) generally have greater electron affinity than atoms from other groups. They require only 1 electron for noble gas configuration.

Off the top of my head the reason there are limits on the numbers of electrons that occupy shells is that shells constitute electron orbitals about a physical space. It's probably not about what actually happens in fact, but the most likely occurrence is for electrons to occupy orbitals of the lowest energy and in doing so you get a sort of organization.

So, probability is my answer.

If you look at the picture on the right of the link below it depicts the kinds of orbitals possible in shells.

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

 

[Redbelly98]

I have known electron affinity as the desire, so to speak, of an atom to bring electrons in its shells. What I don’t understand is why classical theory can’t explain/predict this property of an atom !??

According to classical theory, shells or orbitals do not even exist.

 

[alxm]

My lecturer writes the following in his lecture notes:
“ Electron affinity: a QM property of atom to attract an electron, and increas[ing] the number
of electrons. [The number of electrons] required to complete the outer electron shell decreases. “

(fixed quote)

A more exact definition would be "The change in total electronic energy from adding an electron to an atom or molecule in vacuum.". (With zero being the energy of a free electron, which is the convention here) Chemically you could say it's another way of representing the reduction potential of the atom/molecule:
X + e^- --> X^-

If the outer electron shell (valence shell) is not filled, which is the case for everything except noble gases, then adding an electron will obviously contribute to filling it. (I do find the quoted statement somewhat misleading though, since even noble gases have an electron affinity)

I have known electron affinity as the desire, so to speak, of an atom to bring electrons in its shells. What I don’t understand is why classical theory can’t explain/predict this property of an atom !??

Classical theory cannot predict any properties of an atom. Classically, stable atoms can't exist.

My question is: why is there a limit to the number of electrons that can occupy the outer (or, in fact any) electron shell of an atom?

The Pauli principle in quantum mechanics states that every electron has to have a different set of quantum numbers (n,l,m,s). These represent the different allowed states of the electrons. A 'shell' is the set of states with a given value of n, a sub-shell is a given l value (s,p,d blocks) and an orbital has a given n,l,m value. (which leaves s for spin, which has two possible values, meaning two electrons per orbital)