| ---R---- isn't unique, since HYDROGEN also matches; ------E- isn't unique, since HYDROGEN and NITROGEN also match. (As a *set* they're unique, since HYDROGEN is needed for the second, which means NITROGEN is needed for the fourth; but it's not the case that each pattern is unique.) For a fourth letter, you can use ---G---- or ---L----. You're out of luck for a seventh letter, since of the twenty-four 8-letter elements, seventeen of them have U as a next to last letter (of course), three have E, and the remaining four have N. |