neon has 3 different isotopes, neon-20, neon-21, and neon-22. the first isotope has a mass 19.992 amu and an abundance of 90.60% . the second isotope (neon-21) has an abundance of 0.21%. the third isotope has a mass 21.991 amu and an abundance of 9.20%. what is the mass of the second isotope?