Many Periodic Tables contain a valence-shell configuration.  Valence electrons are contained in the outermost shell or energy level, of an atom.  They are important because they can help predict how that an element reacts chemically with other elements. The outer valence shell can be predicted by the way the Periodic Table of the Elements is constructed.   In Lesson 2 you will learn more about these shells, but for now we just want to read them on the table.   If you look at Iodine on the table is written as (Kr)4d¹⁰5s²5p⁵. The Kr refers to the structure of Krypton, an element in Period 4.   If you look at Krypton’s valence-shell configuration it contains and Ar (Argon) before the structure and if you look at argon it has Ne (Neon) in its formula.    The Neon does not have He (Helium) written before it, but that is because there is room to write helium’s structure of 1s².  So back to Iodine, its entire formula would be written as:  1s²2s²2p⁶3s²3p⁶3d¹⁰4s²  4p⁶4d¹⁰5s² 5p⁵.  That is too much to put in one box.   

Notice that there is a pattern developing.  There are “5” shells, and each shell has a sequence of a s, p, and d.   Complicated?  Only if you don’t see the pattern!  Let’s look at the table closely. Group 1 (alkali metals) and 2 (alkaline earth metals) contains its valence shell in the “s” section.  Group 3-12 (transitional metals) contains the valence electrons is both the “s” and “d” section.  There are a few exceptions, but most of the transitional metals have two electrons in the outer shell.   Groups 13-18 have their valence electrons in the “p” section. The Lanthanide and Actinide Series all contain the “f” section.  However Lanthanum and Actinium do not.

The Periodic Table can predict the valence electrons.  yellow are "s"; gray are "d" generally; blue is "p" and purple are "f".