The Diagonal Rule
The diagonal rule was developed to help us remember the order
in which energy level sub-shells fill, according to the Aufbau
principle (submitted by Kyle S., class of '00.)
The Diagonal Rule:
To use the diagonal rule, follow
the arrows. According to the aufbau principle, electrons will fall into
the shells of lowest energy first. According to the diagonal rule, the
1s sublevel has the lowest energy. So, hydrogen's electron configuration
will be 1s1. Helium's two electrons are shown as 1s2. Now the first energy
level is full and lithium's third (outer) electron will inhabit the 2s
sublevel and is shown as 1s22s1. An observer will notice that the 4s sublevel
will begin to fill before the 3d sublevel. Here is the electron configuration
for scandium: 1s22s22p63s23p64s23d1.