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# Aufbau Principle

## Describes electron configuration within an atom.

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Aufbau Principle

Credit: Gary Minnaert
Source: http://commons.wikimedia.org/wiki/File:LACMA_BCAM02.jpg

#### How are buildings constructed?

Construction of a building begins at the bottom. The foundation is laid and the building goes up step by step. You obviously cannot start with the roof since there is no place to hang it. The building goes from the lowest level to the highest level in a systematic way.

### Aufbau Principle

In order to create ground state electron configurations for any element, it is necessary to know the way in which the atomic sublevels are organized in order of increasing energy. Figure below shows the order of increasing energy of the sublevels.

Credit: CK-12 Foundation - Christopher Auyeung

Electrons are added to atomic orbitals in order from low energy (bottom of graph) to high (top of graph) according to the Aufbau principle. Principal energy levels are color coded, while sublevels are grouped together and each circle represents an orbital capable of holding two electrons.[Figure2]

The lowest energy sublevel is always the 1s sublevel, which consists of one orbital. The single electron of the hydrogen atom will occupy the 1s orbital when the atom is in its ground state. As we proceed with atoms with multiple electrons, those electrons are added to the next lowest sublevel: 2s, 2p, 3s, and so on. The Aufbau principle states that an electron occupies orbitals in order from lowest energy to highest. The Aufbau (German: “building up, construction”) principle is sometimes referred to as the “building-up” principle. It is worth noting that in reality atoms are not built by adding protons and electrons one at a time and that this method is merely an aid for us to understand the end result.

As seen in Figure above, the energies of the sublevels in different principal energy levels eventually begin to overlap. After the 3p sublevel, it would seem logical that the 3d sublevel should be the next lowest in energy. However, the 4s sublevel is slightly lower in energy than the 3d sublevel and thus fills first. Following the filling of the 3d sublevel is the 4p, then the 5s and the 4d. Note that the 4f sublevel does not fill until just after the 6s sublevel. The Figure below is a useful and simple aid for keeping track of the order of fill of the atomic sublevels.

Credit: CK-12 Foundation - Christopher Auyeung

The Aufbau principle is illustrated in the diagram by following each red arrow in order from top to bottom: 1s, 2s, 2p, 3s, etc.[Figure3]

### Summary

• The Aufbau principle gives the order of electron filling in an atom.
• It can be used to describe the locations and energy levels of every electron in a given atom.

### Review

1. What is the Aufbau principle?
2. Which orbital is filled after the 2p?
3. Which orbital is filled after 4s?
4. Which orbital is filled after 6s?

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