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
1s^{2}, and _{} would become
2p^{6}.

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

_{}

to the shorhand version:

1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{1}

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

1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{10} 4p^{6} 5s^{2} 4d^{10} 5p^{6}
6s^{2} 4f^{14} 5d^{9}

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] 6s^{2} 4f^{14} 5d^{9}

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