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# The Aufbau Principle - Electronic Structure and the Aufbau Principle

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The Aufbau Principle - Notation and Exceptions to the Rule

Orbital Trends of the Periodic Table

Todd Helmenstine
The notation seen on period tables for electron configurations uses the form:

nOe

where

n is the energy level
O is the orbital type (s, p, d, or f)
e is the number of electrons in that orbital shell.

For example, oxygen has 8 protons and 8 electrons. The aufbau principle has the first two electrons would fill the 1s orbital. The next two would fill the 2s orbital leaving the remaining four electrons to take spots in the 2p orbital. This would be written as

1s22s2p4

The noble gases are the elements that fill their largest orbital completely with no leftover electrons. Neon fills the 2p orbital with its last six electrons and would be written as

1s22s2p6

The next element, sodium would be the same with one additional electron in the 3s orbital. Rather than writing

1s22s2p43s1

and taking up a long row of repeating text, a shorthand notation is used

[Ne]3s1

Each period will use the notation of the previous period's noble gas.

The aufbau principle works for nearly every element tested. There are two exceptions to this principle, chromium and copper.

Chromium is element 24 and according to the aufbau principle, the electron configuration should be [Ar]3d4s2. Actual experimental data shows the value to be [Ar]3d5s1.

Copper is element 29 and should be [Ar]3d92s2, but it has been to be determined to be [Ar]3d104s1.

The graphic shows the trends of periodic table and the highest energy orbital of that element. It is a great way to check your calculations. Another method of checking is to use a periodic table that has this information on it already.

##### Further Reading on Electron Configurations

Anne Marie Helmenstine, Ph.D.