Average atomic mass from isotopes

Answer Key - Grade 10 Science Worksheet - Building Confidence

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

1. 1.

   Bromine is 50.69% bromine-79 (78.918 u); the rest is bromine-81 (80.916 u) (NIST). What is bromine's average atomic mass, to 0.01 u, Bo?

   79.9 u
2. 2.

   Lithium is 7.59% lithium-6 (6.015 u); the rest is lithium-7 (7.016 u) (NIST). What is lithium's average atomic mass, to 0.01 u, Esme?

   6.94 u

3. Read this first

   Chlorine's isotopes (NIST: IUPAC 2009 abundances, AME2012 masses)

   isotope	protons	neutrons	mass (u)	abundance (%)
   chlorine-35	17	18	34.969	75.76
   chlorine-37	17	20	36.966	24.24

4. 3.

   Using the table, what do chlorine-35 and chlorine-37 have in common, Jun?

   • The same mass
   • 18 neutrons
   • 17 protons
5. 4.

   Using the table, which chlorine isotope is more common, Kira?

   • Chlorine-37
   • They are equally common
   • Chlorine-35
6. 5.

   Using the table, about what is chlorine's average atomic mass, Luz?

   • About 35.45 u
   • About 35.97 u
   • About 36.97 u
7. 6.

   What makes two atoms isotopes of the same element, Milo?

   • Same protons, different neutrons
   • Same mass, different charge
   • Different protons, same neutrons
8. 7.

   Lithium-6 and lithium-7 both have 3 protons. How do they differ, Noah?

   • Lithium-7 has a charge
   • Lithium-7 has one more proton
   • Lithium-7 has one more neutron
9. 8.

   Why do isotopes of an element react in the same way, Pia?

   • They have no neutrons
   • They have the same electrons
   • They have the same mass
10. 9.

    Why is chlorine's atomic mass 35.45 and not a whole number, Ren?

    • It averages two isotopes
    • It was measured badly
    • Its protons weigh 0.45 u extra
11. 10.

    What does the atomic mass on the periodic table describe, Tala?

    • A weighted average of natural isotopes
    • The mass of its heaviest natural isotope
    • The number of neutrons