They're not so different. Both are stable isotopes. But there are far more 16O than 18O. 99.757% of the oxygen atoms in the world are 16O, and 0.205% are 18O.
Oxygen-16 has 8 neutrons and 8 protons. Oxygen-17 has 9 neutrons and 8 protons. The only difference is the atomic mass due to the different number of neutrons. Number of protons & electrons stay...