Which groups of elements are not included in the periodic table ? Explain whether or not you think it is likely that an alien planet would lack these elements.

18 group.

The group 18 elements of periodic table never forms a ionic bond. Only this group is the exception where all the elements present in the group does not form any ionic bond. There are other elements also that do not form ionic bonds but not complete group. The noble atoms have complete outer shell so they can neither lose or gain any electron which form the group 18.