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 6th row of the periodic table:
This means we’re allowed to start from xenon (Xe) at the end of the previous (5th) 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.