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.

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