The
Aufbau Process is the process by which atoms become increasingly more electronically complex by adding one
proton and one
electron. This process parallels the
periodic table in electronic configuration and can be used to determine the number of valence
electrons an
element has. As one reads the
table from
left to
right, atoms become more and more
complex by the addition of one
proton and one
electron. For
groups 1A through 8A (I am excluding the
inner transition elements for now), this creates a reliable pattern by which the number of valence shell
electrons can be determined very quickly. Simply look at the
number before the
A, and you have the number of valence
electrons. This will be in the form of ns
1-2p
1-6, where n is the
period and the s and p orbitals are filled in order from lowest energy to highest energy (between s and p orbitals of the same principal
quantum number, n, the
p orbital will be of the higher energy level and be filled after the
s orbital).
For instance, Neon (Ne(10)) resides in the second period and is in group 8A. It has an electronic configuration of 1s22s22p6. Notice that it has 8 valence electrons, and its highest principal quantum number is 2. Chlorine (Cl(17)) resides in the third period and in group 7A. It has an electronic configuration of 1s22s22p63s23p5. Notice how it has 7 valence electrons and its highest principal quantum number is 3.