Neon has three naturally occurring isotopes: Ne-20 with mass 19.9924 amu and a natural abundance of 90.60 %, Ne-21 with mass 20.9938 amu and a natural abundance of 0.260 %, and Ne-22 with mass 21.9914 amu and a natural abundance of 9.20 %

20.191 a. m. u

Explanation:

To calculate the average atomic mass for neon, we take into consideration the individual isotopes and their masses. We multiply the percentage of each of their abundances by their masses and add the results together to arrive at the value.

This can be done as follows;

[90.60/100 * 19.9924] + [0.26/100 * 20.9938] + [9.2/100 * 21.9914] = 18.113 + 0.055 + 2.023 = 20.191 a. m. u