Answer
${\bf 20.18}\;\rm u$
Work Step by Step
To calculate the chemical atomic mass of neon, we need to take into account the isotopic masses of neon and their natural abundances.
Neon has three main isotopes:
1. Neon-20 ($^{20}\text{Ne}$) with a natural abundance of approximately 90.48% and an atomic mass of 19.99244 u.
2. Neon-21 ($^{21}\text{Ne}$) with a natural abundance of approximately 0.27% and an atomic mass of 20.99385 u.
3. Neon-22 ($^{22}\text{Ne}$) with a natural abundance of approximately 9.25% and an atomic mass of 21.99139 u.
Thus, the atomic mass of Neon is given by
$$m_{\rm Ne}=0.9048
m_{^{20}\text{Ne}}+0.0027 m_{^{21}\text{Ne}}+0.0925
m_{^{22}\text{Ne}}$$
Plug the known from Appendix C;
$$
m_{\rm Ne}=0.9048
(19.992 435
)+0.0027 (20.993 841
)+0.0925 (21.991 383)
$$
$$m_{\rm Ne}=\color{red}{\bf 20.18}\;\rm u$$
