How do we go about remembering the sequence in which electrons
fill the sublevels?
The order in which electrons fill
the sublevels is easy to remember if you follow these steps:
Write the principal energy levels and their sublevels on separate
lines (as shown on the diagram).
Draw arrows over the sublevels (see the red diagonal lines on the
diagram by placing your mouse over the diagram).
Join the diagonal lines from end to end (click on the diagram to see
how I have joined the red diagonal lines).
Find the "start here" arrow and follow the arrows. The sublevels
are magically arranged in the correct sequence from lowest energy (i.e.
compare the order of filling sublevel sequence with the energy
diagram of the sublevels).