I believe you are referring to one of the many listed ways to define https://oeis.org/A001105 , which is even more succinctly expressed as a(n) = 2*n^2. This is the answer I thought you wanted, and which I was avoiding saying.
My underlying point is your sequence was under-specified, and with no clear reason to pick your preferred answer over other answers.
Even knowing it's chemistry, three finite sequences which match your specification are:
My underlying point is your sequence was under-specified, and with no clear reason to pick your preferred answer over other answers.
Even knowing it's chemistry, three finite sequences which match your specification are: