Indicates the number of
electrons in each
subshell. It contains a number, which stands for the
principal energy level, followed by a letter, which is the
subshell, followed by a
superscript numeral which is the number of electrons found in that subshell.
Example: 1s2 2s2 2p6 3s2 3p6 is the electronic configuration of Argon. The sum of all the superscripts is 18, which equals the number of electrons which Argon has, which is equal to it's atomic number.
The principal energy levels are numbered starting at 1, and increases by one for each level. The sublevels are named in this order s, p, d, f, g, h, i, etc. (all in alphabetical order after f, but it's a moot point considering h, i, etc. don't even exist, neither naturally nor artificially (yet)).
The number of subshells each principal energy level has (also known as 'shell') is equal to the number which names it. The 1st shell has one subshell (s). 2nd shell has two subshells (s,p). 3rd shell has 3 subshells (s, p, d), etc.
Each subshell can hold a limited number of electrons. The nth subshell can hold 4n - 2 number of electrons. Therefore the 's' subshell (1st) holds two (4*1 - 2), the 'p' subshell (2nd) holds 6 (4*2 - 2), d = 10 electrons, f = 14 electrons, etc.
When one reaches the higher levels the order of the shells and subshells can get confusing. A shell can start filling before the last one finished.
The order of the subshells can be found by creating the following chart.
1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f 5g
6s 6p 6d 6f 6g 6h
7s 7p 7d 7f 7g 7h 7i
In order to find the order you must imagine, or draw, arrows coming from the top right to the bottom left going through the last sublevel on each row, and the sublevel below it (on the chart). The order which is found when consulting the chart is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, etc.