Let us start with sodium with one electron in the outside shell. The M shell fills its s and p subshells with eight electrons. Argon, element eighteen, has two electrons in the K shell, eight in the L shell, and eight in the M shell. The fourth period begins again with potassium and calcium, but a different pattern appears here. After the addition of the 4s electrons and before the addition of the 4p electrons, the sequence goes back to the third energy level to insert electrons in a d shell.

The shells or energy levels are numbered or lettered, beginning with K. So K is one, L is two, M is three, N is four, O is five, P is six, and Q is seven. As the s shells can only have two electrons and the p shells can only have six electrons, the d shells can have only ten electrons and the f shells can have only fourteen electrons. The sequence of addition of the electrons as the atomic number increases is given here. The first number is the shell number, the s, p, d, or f represents the type of subshell, and the last number shows the number of electrons in the subshell.

One way to remember this sequence is to memorize it. There is a bit of a pattern in it. Another way to learn this sequence is to look at the periodic table. As you go from hydrogen down the chart, the Groups 1 and 2 represent the filling of an s subshell. The filling of a p subshell is shown in Groups 3 through 8. The filling of a d subshell is represented by the transition elements (ten elements), and the filling of an f subshell is shown in the lanthanide and actinide series (fourteen elements).

BASIC ORBITAL SHAPES

The shape of the s subshells is spherical. The shape of the p subshells is the shape of three barbells placed at ninety degrees to each other. The shape of the d and f subshells is very complex with a spatial distribution.It can only be visualized in 3-D.

Electron configuration is the "shape" of any electron around an atom. This refers to the energy level (shell) and the kind of orbital it is in. The shells were historically named for the chemists who found and calculated the existence of the first (inner) shells. Their names began with "K" for the first shell, then "L," then "M," so subsequent energy levels were continued up the alphabet.

The electron configuration is written out with the first number written in large font being the shell number. The letter is the orbital type (either s, p, d, or f). The smaller superscript number is the number of electrons in that orbital.

This scheme must be used as follows. You first must know the orbitals. An s orbital only has 2 electrons. A p orbital has six electrons. A d orbital has 10 electrons. An f orbital has 14 electrons. You can tell what type of orbital it is by the number on the chart. The only exception to that is that "8" on the chart is "2" plus "6," that is, an s and a p orbital.

The chart reads from left-to-right and then down to the next line, just as English writing. Any element with over 20 electrons in the electrically neutral unattached atom will have all the electrons in the first row on the chart. For instance, scandium, element #21, will have all the electrons in the first row and one from the second. The electron configuration of scandium is: 1s2 2s2 2p6 3s2 3p6 4s2 3d1 Notice that the 2s2 2p6 and 3s2 3p6 came from the eights on the chart (2+6). Notice that the other electron must be taken from the next spot on the chart and that the next spot is the first spot on the left in the next row. It is a 3d spot due to the "10" there and only one more electron is needed, hence 3d1.