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Every isotope has integer mass number ... proton number + neutron number
the proton number determines what element they are ... that is the atomic number.
      example : Neon , in the right-most column , is atom number 10 (green)
. . . Any particular element might have a variety of neutron numbers
      having extra neutrons or not quite so many as the most common kind.
      most often, having 2 extra neutrons, or lacking a pair of neutrons.

The average atomic mass is listed on our Periodic Table of Elements directly under the symbol.
step 1) round the atomic mass (down or up) to the closest integer
      that is (almost always) the most typical isotope for that element (name).
      the average Neon atom's mass is 20.18
      which rounds down to 20 as the most common isotope mass.

step 2)
. . . If the average atom is heavier than the most common isotope,
      it must be that some of those atoms are heavier than usual.
      most ₁₀Ne atoms have mass 20 , but rare ones have mass 22
. . . If the average atom is lighter than the most common isotope,
      then some of those atoms must be lighter than usual
      ₅Boron (in blue) is mostly boron-11 , but some are boron-10

step 3)
however much was rounded off to get to the integer determines how much of the rare kind a sample has
we had to add 0.19 to boron's 10.81 to get up to the 11 . . . but neon is 0.18 too heavy
. . . so about 19% of boron is the lighter-mass boron-10 . . . 0.19 / 1 = 19%
. . . because the rare neon has 2 extra neutrons, heavy neon is 0.18 / 2 = .09 = 9% of natural neon.

Look at ₁₈Ar to check your understanding.
1) which kind is most common?
2) is the rare kind heavy or light?
3)a) what % of an Ar bottle would be Ar-39?
3) there is no Ar-39 ... it is Ar-36 ... what % is that?