Hi!
I believe there are 4 main chemicals in space uracil, cytosine, thymine, and pyrimidine :)
The mass of an element listed in the Periodic Table is the weighted average of all its naturally occurring isotopes.
Naturally occurring carbon is about
99 % carbon-12 (12.000 u) + 1 % carbon-13 (13.003 u).
That extra carbon-13 makes the <em>average atomic mass</em> greater than 12.000 u.
