Question

The three stable isotopes of neon ${}_{10}N{e}^{20},{}_{10}N{e}^{21}$ and ${}_{10}N{e}^{22}$ have respective abundances of $90.51\%,0.27\%$ and $9.22\%$. The atomic masses of the three isotopes are $19.99u$, $20.99u$ and $21.99u$ respectively. The average atomic mass of neon is

• A. 11.18 u
• B. 15.18 u
• C. 20.18 u
• D. 10.18 u

Answer 1

Answer: (C)

The masses of three isotopes are $19.99$ u, $20.99$ u, $21.99u$
Their relative abundances are $90.51\%,0.27\%$ and $9.22\%$
$\therefore$ Average atomic mass of neon is
$m=\frac{90.51\times 19.99+0.27\times 20.99+9.22\times 21.99}{(90.51+0.27+9.22)}$
$=\frac{1809.29+5.67+202.75}{100}=\frac{2017.7}{100}=20.18u$