The positions of the elements La, Ac, Lu, and Lr in the WebElements periodic table is justified in the article by William B. Jensen, J. Chem. Ed., 1982, 59, 634.

Elements 117, and 118 are not known at the time of writing, but are shown "greyed-out" in the tables at their expected positions for information.

Periods in the periodic table

There are 7 periods in the standard form of the periodic table:

• Period 1
• Period 2
• Period 3
• Period 4
• Period 5
• Period 6
• Period 7

Period Number

In the standard form of the periodic table the s-block, p-block, and d-block elements are organised into horizontal rows called periods. In WebElements these are labelled from 1 to 7.

The f-block

The f-block elements are assigned to Periods 6 (lanthanoids) and 7 (actinoids) since that is where they are located in the full, or extended, version of the periodic table.