The Final Four Periods

The next two periods, four and five, each add another eight elements to the Periodic Table filling the 4s and 4p, and the 5s and 5p shells respectively. But in addition to this, they each also fill a new orbital, the d shell. Each d shell contains ten more electrons and is filled after the s shell. For the fourth period the first two elements, Potassium and Calcium, fill the 4s shell. Then come ten transitional metals filling the 3d shell followed by six elements filling the 4p shell. The fifth period follows the same pattern.

Not all of the elements in the fourth period have biological activity. Potassium (K - 19) and Calcium (Ca - 20) are essential to life. Of the ten transitional metals, most are considered trace elements or minerals (as consumed naturally) with the exception of Iron (Fe - 26) which is essential for Oxygen transport and cellular respiration. The remaining six elements are also mostly biologically active to a small degree with the exception of Arsenic, which is extremely toxic, and Krypton which is an un-reactive Noble Gas.

In the fifth period most of the elements are only minimally biologically active with the exception of Iodine (I - 53). Several are considered trace elements which only appear to be needed when completely missing from the diet as when someone is feed intravenously with Total Parenteral Nutrition.

The sixth period contains most of the remaining natural elements. It also includes the inner transitional metals which have electrons filling the f shell. This shell contains 14 electrons. The seventh period is similar except that most of the elements are synthetic, existing only for extremely short half lives when generated by nuclear reactors or particle accelerators.

This completes the Periodic Table up to element 118, the highest discovered element, Ununoctium, a Group 18 element (Noble Gases). Only 3 to 4 atoms of this element have ever been produced so it is not known if it is a gas or has properties like those of the other Group 18 Noble Gases.

In the next series of articles I will cover some of the more well known elements in periods 4 through 7 as well as some interesting facts about the lesser known ones before going back to period 2 and looking at Carbon, Nitrogen, Oxygen and Hydrogen (period 1).