Understanding Electron Configuration Tables
What is an electron configuration table?
An electron configuration table is a convenient way of representing how many electrons are in each energy level of an atom and how these electrons are arranged within each energy level. At first it may seem complicated, but once you understand what each part means it becomes much easier to interpret.
What does it all mean?
There are two ways of representing the electron configuration, the following two examples are for the element samarium;
The table on the left shows the seven possible energy levels (the rows) that an electron can be in, it also shows the sub-shells that make up each energy level (the columns); p, s, d and f. Notice how not all energy levels have the same available sub-shells. Each coloured rectangle represents one electron (the colours are there to illustrate increasing energy only).
The table on the right is showing the same information just in a slightly different way.
The format of the table is: [energy level][subshell][number of electrons in subshell].
[energy level] From the table we can see that samarium has electrons in six energy levels.
[subshell] The sub-shell where the electrons reside, either; s,p,d or f.
[number of electrons in subshell] The s sub-shell can only hold two electrons, the p sub-shell can hold 6, the d sub-shell can hold 10 and the f sub-shell can hold up to 14. Notice that not all of the sub-shells have to be full before electrons can move to the next available energy level.
Note that either representation of the electron configuration of this atom can also be written as;
2,8,18,24,8,2 (the total number of electrons in each level)
or
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f6 5s2 5p6 6s2
where each part of the electron configuration is written out in sequence, this is usually abbreviated to;
[Xe] 4f6 6s2
This means that the electron configuration is the same as that of xenon but with six extra electrons in the f sub-shell of the 4th energy level (4f6) and 2 extra electrons in the s sub-shell of the 6th energy level (6s2).