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 ns1-2p1-6, 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.

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