Electron Configuration Notations

So far, we have represented the electron configuration of an atom using orbital notation, with a diagram like the following one for aluminum:

In this diagram:

· the numbers 1, 2, 3, ... under the blanks represent the energy level

· the letters s, p, d, and f represent the sub-level

· each blank represents an orbital

· each arrow represents an electron

The advantage of using this notation is that it gives detailed information about every electron in the atom.  The disadvantage is that it’s tedious to write.

If you don’t need to draw every electron, you can use a shorter form, in which you just write the level and sub-level, and use an exponent for the number of electrons in the sub-level.  For example,  would become 1s², and  would become 2p⁶.

The electron configuration for aluminum would go from the orbital notation version:

to the shorhand version:

1s² 2s² 2p⁶ 3s² 3p¹

The shorter version can still get tediously long for elements with a lot of electrons.  For example, the electron configuration for gold (Au) is:

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d⁹

To shorten this even more, you’re allowed to use the element in the last column of a row as an abbreviation for all of the electrons through the end of that row.

In our example, gold (Au) is in the 6^th row of the periodic table:

This means we’re allowed to start from xenon (Xe) at the end of the previous (5^th) row, and add on the parts that come after Xe.  This gives us:

[Xe] 6s² 4f¹⁴ 5d⁹

This notation is called the noble gas configuration, because the elements in the last column are the noble gases.