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Auf Bau Build Up Principle


The Auf Bau buildup principle is used to determine the electron configuration of an atom. The electron configuration lists the subshells containing electrons, and how many electrons are in each subshell. In the example below, neon, there are ten electrons, with 2 in the 1s subshell, 2 in the 2s subshell, and 6 in the 2p subshell.


Auf Bau buildup assumes that one "builds" an atom by starting with a nucleus (protons and neutrons), and then adding electrons, one at a time, to electron subshells, given designations like 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, etc.

Subshells designated 's' contain 2 electrons.
Subshells designated 'p' contain 6 electrons.
Subshells designated 'd' contain 10 electrons.

Other subshells exist, but we will limit our discussion to the above. The number of electrons in each subshell is designated by a superscript to the right of the subshell. The order of subshells will be shown below, followed by a listing of atoms and their electron configurations.

The order of the series of subshells is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s,

A triangular matrix can be drawn, as shown below, and then by drawing arrows in the manner shown in the animated gif, one can derive the above series:


It is possible colorize the periodic table in ways to associate atoms with the different subshells, s, p, d, etc. The atoms in red has an s for their highest subshell, orange for p, and yellow for d. Why is helium placed over neon? 





H - 1s1
He- 1s2
Li- 1s22s1
Be- 1s22s2 
B - 1s22s22p1
C - 1s22s22p2
N - 1s22s22p3
O - 1s22s22p4
F - 1s22s22p5
Ne- 1s22s22p6
Na- 1s22s22p63s1
Why 'build-up?' In writing out electron configurations, you move from left to write, but when you fill in orbitals in an energy diagram, you move from bottom to top. 1s is the lowest level, 2s is the second level, 2p is the third level, 3s is the fourth level, 3p is the fifth level, 3d is the sixth level, etc.





In looking at the table, one might question, "Why helium is placed over neon, when, according to the subshells of electron configuration, it should be placed over beryllium?"



Helium has more in common with neon, argon, krypton, and xenon, than it does with beryllium, magnesium, calcium, etc.





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Last Revised 01/25/98.
Copyright ©1998 by William L. Dechent. All rights reserved.