ANSWER:
Neon has three isotopes Ne–20, Ne–21 & Ne–22, having percentage abundance of 90.92%, 0.26% & 8.82% respectively. When we calculate average of all these isotopic masses in proportion to their natural abundances, a fractional value of 20.18 amu is obtained that is called the average atomic mass of Neon.
![Chem-11, Q 5a](https://exceptionaleducations.com/wp-content/uploads/2022/12/Chem-11-Q-5a-1024x152.png)
So, 20.18 amu is not the mass of any individual Neon atom. Rather, it is the average or relative atomic mass of Neon.
(Ref. An Insight Into Objective Chemsitry-11, Ch-01, Q. No. 27)