The positions of the elements La, Ac, Lu, and Lr in the WebElements periodic table is justified eloquently in the article by William B. Jensen.1 in J. Chem. Ed..

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

• 1. The positions of lanthanum (actinium) and lutetium (lawrencium) in the periodic table,
  
  , Journal of Chemical Education, 08/1982, Volume 59, Issue 8, p.634-636, (1982)
  

The positions of lanthanum (actinium) and lutetium (lawrencium) in the periodic table, , Journal of Chemical Education, 08/1982, Volume 59, Issue 8, p.634-636, (1982)

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.