The diagonal rule is a method for determining the order in which electrons fill sublevels of energy levels of atoms when doing electron configurations. The numbers follwing the letters should be superscript, but I cannot do that in context.
/\
|\/ \
| 1s2 \
| /
| /
|____\
/\ /\
|\/ \ / \
| 2s2 \ /2p6 \
| / / /
| / / /
|____\ / /
/ /
/ /
/ /
/ / /\ /\
|\/ / / \ / \
| 3s2 / /3p6 \ /3d10\
| / / / / /
| / / / / /
|____\ / / / /
/ / / /
/ / / /
/ / / /
/ / / / /\ /\
|\/ / / / / \ / \
| 4s2 / /4p6 / /4d10\ /4f14\
| / / / / / / /
| / / / / / / /
|____\ / / / / / /
/ / / / / /
/ / / / / /
/ / / / / /
/ / / / / / /\
|\/ / / / / / / \
| 5s2 / /5p6 / /5d10 / /5f14\
| / / / / / / /
| / / / / / / /
|____\ / / / / / /
/ / / / / /
/ / / / / /
/ / / / / /
/ / / / / / /\
|\/ / / / / / / \
| 6s2 / /6p6 / /6d10 / /6f14\
| / / / / / / /
| / / / / / / /
|____\ / / / / / /
/ / / / / /
/ / / / / /
/ / / / / /
/ / / / / / /\
|\/ /|\/ /|\/ /|\/ \
| 7s2 / | 7p6 / | 7d10 / | 7f14\
| / | / | / | /
| / | / | / | /
|____\ |____\ |____\ |____\
You start with the top arrow (1s2), then the one below that (2s2), then the 2p6 arrow, which also includes 3s2, then you use the 3p6 arrow...
Plutonium, element 94:
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f6
Note how I ended on 5f6, not 5f14. I ran out of electrons, so I couldn't fully fill that sublevel. If you add up all the superscripts, you will indeed come up with 94. | 
