Average atomic mass from isotopes

Answer Key - Grade 10 Science Worksheet

Circle the best answer, or write your answer on the line.

1. 1.

   Boron's average atomic mass is 10.81 u. Its isotopes are boron-10 (10.013 u) and boron-11 (11.009 u). What percent of boron is boron-10, to 0.01%, Ari?

   19.98 %
2. 2.

   Silver's average atomic mass is 107.868 u. Its isotopes are silver-107 (106.905 u) and silver-109 (108.905 u). What percent of silver is silver-107, to 0.01%, Esme?

   51.85 %
3. 3.

   Copper's average is 63.546 u, between 62.930 u and 64.928 u. Which isotope is more common, Jun?

   • Copper-65
   • They are equal
   • Copper-63
4. 4.

   Silver's average, 107.868 u, sits near the middle of 106.905 u and 108.905 u. What does that suggest, Kira?

   • Silver-109 makes up nearly the entire sample
   • Silver has one isotope
   • The isotopes are about equally common
5. 5.

   NIST gives boron's standard atomic weight as a range, 10.806 to 10.821. Why, Luz?

   • Boron atoms change size from day to day
   • Boron hasn't been weighed yet
   • Natural samples vary in isotope mix
6. 6.

   Why is a plain average of isotope masses wrong for an element, Milo?

   • The isotopes aren't equally common
   • Isotopes have equal masses
   • Averages can't use decimal numbers
7. 7.

   When would a plain average of two isotope masses give the right answer, Noah?

   • When both are 50% of the element
   • When the masses are whole numbers
   • When one is 100% of it
8. 8.

   How is a percent used in the weighted average, Pia?

   • As a fraction, like 0.7576
   • It is added to the mass
   • As a whole number, like 76
9. 9.

   What fraction of a radioactive isotope is left after two half-lives, Ren?

   • One quarter
   • Nothing left
   • One half
10. 10.

    A sample falls from 80 g to 20 g of an isotope. How many half-lives have passed, Zee?

    • Two
    • Three
    • Four