It is more appropriate to talk about shells than energy levels, due to complexity of atoms beyond hydrogen.
Each shell has a maximum number of electrons that it can hold. For a higher value of \(n\), the shell is further from the nucleus and has greater energy.
| First Shell | \(n=1\) | 2 electrons |
| Second/Third Shells | \(n=2\)/\(n=3\) | 8 electrons |
| Fourth/Fifth Shells | \(n=4\)/\(n=5\) | 18 electrons |
| Sixth/Seventh Shells | \(n=6\)/\(n=7\) | 32 electrons |
The lowest energy shells are filled first. Much of chemistry is decided by the outer shell electrons.
Sub-Shells are labelled s, p, d, f. These correspond to the shells:
\(n=1\) has an s sub-shell
\(n=2\) has s and p
\(n=3\) has s, p, and d
\(n=4\) has s, p, d, and f
| Sub-Shell | Maximum number of electrons |
| s | 2 |
| p | 6 |
| d | 10 |
| f | 14 |
In atoms other than hydrogen, sub-shells within a shell have different energies. The shells of 3d and 4s have an overlap in energies.
S sub-shells have one s-orbital
P sub-shells have three p-orbitals
D sub-shells have five d-orbitals
F sub-shells have seven f-orbitals
In an isolated atom, orbitals within the same sub-shell have the same energy.
Each orbital can hold a max of 2 electrons
Must have opposite spin
Corresponds to clockwise or anti-clockwise
The position of an electron is mapped with a probablilty function as it cannot be pinpointed exactly.
The orbitals are filled to give the lowest energy arrangement possible. To do so, they are filled in order of increasing energy.
Orbitals will take one electron until all are full
4-s fills before 3-d as it is lower energy
4-s also empties first
Not required to write electron configs for Copper or Chromium.
Eg. Scandium: 1s22s22p63s23p63d14s2