I have been taught in school that atoms cannot have more than 8 electrons in the outer shell. Palladium atom's electron configuration is 2,8,18,18. Why isn't it 2,8,18,17,1 like the case of Platinum 2,8,18,32,17,1 or Nickel 2,8,17,1 and they are all in the same group?
As suggested by M.Sameer I convert my comment into an answer:
Dear M.Sameer: It seems that you are missing that the $n+\ell$ Madelung rule is not an exact result derived from first principles, but rather a rule of thumb, that holds for, say, approximately 95 percent of all the elements, with important exceptions, cf. this wikipedia page. Nevertheless, there are semi-rigorous theoretical arguments why such a rule of thumb should hold to a good approximation.
all d-block elements will have their last electron in 'd' orbital. thus even if it is in the 5th period it has no 5th shell as it first fills the 'd' block in 4th shell