Internet Database of Periodic Tables

Aufbau principle or "electrons in boxes" as my AS syllabus so annoyingly puts it.

I was confused at first, but a quick wikipedia search showed that Madelung's rule is also known as the Aufbau principle, which is how I've seen it named throughout the southeastern US.

Ah, bringing back fond memories of high school chem. I loved this stuff.

Madelungs rule lets you know -how- the electrons are likely to combine. There is the elementary chemistry of simply stating an element has available plus or minus a certain free electrons, which is simpler and roughly works in a lot of simple situations. this just digs a bit deeper into the different orbitals, the energy states of the electrons.

It is specifically necessary for proper understanding of Organic Chemistry, where the shape of the molecule is often based on these electron energy states. Carbon can have several charge states, can offer or accept one or several electrons, and the orbit and energy state of the electrons will let you know the angle that the other ions may be likely to attach, whether a carbon based molecule is likely to have a ring structure, or be linear, etcetera.