@@ -116,11 +116,11 @@ The order of orbital filling is set by several rules:
* _Aufbau principle_: electrons first fill a complete shell (all electrons with the same $n$) before going to the next one
* _Madelung's rule_: electrons first occupy the shells with the lowest $n+l$. If there are several orbitals with equal $n+l$, electrons occupy those with smaller $n$
Combining the two rules, we obtain the shell filing order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, etc.
Combining the two rules, we obtain the shell filling order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, etc.
While these rules accurately predict the electronic structure of most elements, they are only approximate, and fail to describe some of the [heavier elements](https://en.wikipedia.org/wiki/Aufbau_principle#Exceptions_in_the_d-block).
Shell filing is important to us because the valence electrons (those in the outermost shell) are the only ones participating in chemical reactions and electric conduction.
Shell filling is important to us because the valence electrons (those in the outermost shell) are the only ones participating in chemical reactions and electric conduction.
From the valence electrons' point of view, the inner shell electrons act like a negatively charged cloud.
The electrostatic repulsion between them reduces the effective charge of the atomic nucleus, but does not play any further role.
