Question

What are shown at the bottom of the periodic table in order to keep the table a reasonable size?

Answer 1

The lanthanides are generally found in the top and bottom rows beneath the main body of the periodic table, the actinides are below them.

Explanation:

That's because the lantganides and actinides woukd require 14 additional columns each having only two elements apiece, meaning the table would have a lot of "dead space" above them.

Lanthanides are so called because they come after lanthanum in atomic number (lanthanum = 57, lanthanides = 58-71) and they have properties more or less similar to lanthanum. Actinides, below the lanthanides in the usual version of the periodic table, have a similar relationship with actinium.