Answer:
Neon-20 would be the most abundant isotope among the three.
Explanation:
The relative atomic mass of an element is the average of the relative isotope mass of its naturally-occurring isotopes, weighted according to the relative abundance of each isotope.
The relative isotope mass of an isotope atom is approximately the same as its mass number. The relative isotope mass of one neon-20 atom is approximately [tex]20[/tex]. Similarly, the relative isotope mass of one neon-21 atom and one neon-22 atom will be [tex]21[/tex] and [tex]22[/tex], respectively.
The relative atomic mass of neon is approximately [tex]20.180[/tex] (as seen on a modern periodic table.) This number is supposed to be the weighted average of the relative isotope mass of neon-20, neon-21, and neon-22 according to their relative abundance.
If the three isotopes were to have a relatively equal abundance, then the relative atomic mass of neon should be very close to [tex]21[/tex]. However, that's not actually the case. The commonly-accepted relative atomic mass of neon, [tex]20.180[/tex], is significantly tilted towards [tex]20[/tex], which is the (approximate) relative isotope mass of neon-20.
The question assumes that neon-20, neon-21, and neon-22 are the only three naturally-occurring isotopes of neon. In other words, there's no naturally-occuring isotope of neon with an isotope mass smaller than that of neon-20. Based on that assumption, neon-20 must have been considerably more abundant than the heavier neon-21 and neon-22 to ensure that weighted average [tex]20.180[/tex] is much smaller than the relative isotope mass of neon-21 and neon-22.
