What is the Aufbau Principle?
Today we’re going to talk a little bit about the Aufbau principle. The Aufbau principle is from a German word “aufbau” which means to “build up” or “construct,” and it’s related to quantum chemistry, (quantum physics) when we talk about building up or constructing an atom.
An atom, of course, made of protons, neutrons, electrons and this particular, or specific, principle has to do with the electrons and what shell they occupy, so you, basically, are building up (as you go up in the atomic weights) the constituent elements from the prior ones (early ones) to later ones.
I’ve got an example on the board, but we first want to see an easy way, a mnemonic, to remember what order they go in. Essentially, you start here with the different energy levels, 1 through 8, and then it follows this pattern here, if you can think of sort of a loop.
The first shell holds 2 electrons and only 2 electrons and it’s 1S, and then it comes back, and the second shell is also an S shell, only holds 2 electrons. It comes down, and then it comes up here to 2P.
The P shell can hold 6, and it goes to 3S back over here to 3P, 4S, and then out to 3D, 4P, 5S. 4D, 5P, 6S all the way out here to 4F, 5D, 6P, 7S. 5F, 6D, 7P, 8S and it follows this pattern. As you go up in the atomic weight, you can follow this pattern and predict, basically, where the electrons will be.
I’ve given us an example here on the board with nitrogen. Nitrogen has 7 protons and 7 electrons, and if we’re going to build up or construct, applying the Aufbau principle, we would need to know what shells those electrons are going to occupy.
If we go to our chart, we realize we’ve got 7 electrons we’ve got to deal with, but the S shell can only hold 2. We put 2 in the 1S, so begin here then with 1S. Come back to the second shell, 2S can hold 2 electrons, so we started with 7, we’ve used 2, we’ve got 5 left. 2 electrons in the 2S, and we’ve got 3 left, which takes us out to 2P.
The P area can hold 6 electrons, so the final 3 go, then, to 2P. This is a way basically to predict where the electrons will go as they fill the various shells. The general idea is that the electron will fill the lowest energy level first prior to moving up to another one, and it’ll fill same spin first, before going to alternate spin.
This has just been just a basic, basic overview of the Aufbau principle, a way to build up or construct atoms predicting where the electrons will be in the shells based on this chart here.