Getting bored of all the sequences (including mine)?
Here is a little different one for you.
What is the last one in the following:
C, H, O, P, S, ?
Tell me why?
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I think the answer is
These are chemical symbols representing the six most important chemical elements whose covalent combinations make up most biological molecules on Earth ("six elements of life"): carbon, hydrogen, oxygen, phosphorus, sulphur, nitrogen
The answer is indeed
N for Nitrogen
but let me take a stab at the title:
$3^2 + 2^3 + 2^2 = 21$; "guesses" more or less sounds as "gas is" so "21 gas is left". Air consists of 79% Nitrogen and 21% other gasses.
This is a long shot, but the title could be a hint towards it.
3, 8, 15, 16, 19, 23, ... is the sequence of numbers whose base-5 representation contains no 2's and exactly one 3. (OEIS.)
According to the answer of hexomino I would say
This is the writing order of chemical formula according to the Hill System, where atoms in molecules have to be named in this particular order.