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)

 

[sardar]

Electron affinity is a quantum mechanical property of an atom that describes its ability to attract and hold an additional electron. This property is influenced by the electronic structure of the atom, specifically the number of electrons in its outermost shell. In general, atoms with a higher electron affinity tend to have a stronger pull on additional electrons, while those with a lower electron affinity have a weaker attraction.

The reason why classical theory cannot explain or predict electron affinity is because it does not take into account the wave-like nature of electrons and their behavior in the atomic structure. Classical theory only considers the movement of particles in a predictable manner, whereas quantum mechanics allows for the concept of electron shells and the probability of finding an electron in a particular location.

As for the statement "increases the number of electrons required to complete the outer electron shell decreases," it means that as the number of electrons in the outer shell increases, the electron affinity of the atom decreases. This is because the additional electrons experience more repulsion from the existing electrons in the outer shell, making it harder for the atom to attract and hold onto another electron.

Regarding the limit to the number of electrons that can occupy the outer shell of an atom, this is due to the concept of shell filling and the Pauli exclusion principle. According to this principle, no two electrons can occupy the same quantum state or have the same set of quantum numbers. As the number of electrons in a shell increases, the available quantum states become more limited, making it energetically unfavorable for additional electrons to occupy that shell.

I hope this explanation helps to clarify the concept of electron affinity and why classical theory cannot fully explain it. For a more in-depth understanding, I suggest further studying the principles of quantum mechanics and atomic structure.