The proper counterpart of "to the left of" is "below", but the entire sentence seems a bit suspect. It should probably be "Atomic radius increases as you move downwards or leftwards on the periodic table," or possibly "Atomic radii increase below or to the left of any given element."
"To the left of the periodic table" implies something on the left of the entire table, which is not what the original sentence is meant to convey.
NOTE: We also would say "above", rather than "to the up of," but I do have a private theory, entirely unconfirmed, that "above" was originally "up of". Maybe I should make that a question and see if any etymologists can confirm or deny.