Isotopes and Relative Atomic Mass · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Isotopes and relative atomic mass

Science · Matter & Materials · ages 14-15
Name ______________________   Date ____________

  1. Why does the chart print 35.5 beside chlorine?

    • one chlorine atom really does weigh 35.5
    • every atom is missing half a neutron
    • the chart rounds all its values to one decimal place
    • it is an average taken across a mixture of isotopes

  2. Why do potassium-39 and potassium-41 behave the same way in every reaction?

    • their electron counts are both 19
    • they share a mass number
    • their nuclei both take part in the reaction
    • they both hold the same number of neutrons

  3. Magnesium-24 and magnesium-26 are written with different numbers after the name. What are those numbers?

    • the protons packed in each nucleus
    • the neutrons packed in each nucleus
    • the electrons out in the shells
    • the mass numbers of the two atoms

  4. Two atoms both have 6 protons, one with 6 neutrons and one with 8. How are they related?

    • Same protons, different neutrons
    • Same neutrons, different protons
    • Same mass, different element
    • Same electrons, different protons

  5. Gallium is 60 per cent gallium-69, and the rest is gallium-71. What is its relative atomic mass?

    Answer: ______________

  6. An element has two isotopes, one of mass number 28 and one of mass number 30. Its relative atomic mass is 28.1. Which isotope is more common?

    • the one with mass number 28
    • the one with mass number 30
    • the two are equally common
    • the figure cannot tell you

  7. Copper is 70 percent Cu-63 and 30 percent Cu-65. Calculate its relative atomic mass.

    Answer: ______________

  8. Bromine is 50 per cent bromine-79 and 50 per cent bromine-81. What is its relative atomic mass?

    Answer: ______________

  9. Ola says chlorine-37 must react more slowly than chlorine-35, because it is the heavier atom. What is wrong with that?

    • chlorine-37 is really the lighter of the two isotopes
    • the two isotopes carry different numbers of electrons
    • heavier atoms always react faster rather than slower
    • reactions are settled by electrons, and both carry 17

  10. Element A has two isotopes split 50 to 50. Element B has two isotopes split 90 to 10. Whose relative atomic mass sits closer to the midpoint of its two isotope masses?

    • element B, because one isotope dominates
    • both land exactly on their midpoint
    • element A, since neither isotope outweighs the other
    • neither, because an average never lands in the middle
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Isotopes and relative atomic mass W1-mt_NFoRZd6UN_-s1

  1. it is an average taken across a mixture of isotopes · Chlorine atoms come in two kinds, 35 and 37. The printed figure describes the mixture, so it can land between them.
  2. their electron counts are both 19 · Both are potassium, so both hold 19 protons and 19 electrons. Reactions are settled by electrons, and the two extra neutrons change none of them.
  3. the mass numbers of the two atoms · The number after the name is the mass number. Magnesium has 12 protons, so magnesium-24 holds 12 neutrons and magnesium-26 holds 14.
  4. Same protons, different neutrons · Isotopes share proton count and differ in neutrons.
  5. 69.8 · The rest is 100 - 60 = 40 per cent gallium-71. Then 60 x 69 = 4140 and 40 x 71 = 2840, adding to 6980, so the answer is 69.8.
  6. the one with mass number 28 · 28.1 sits almost on top of 28 and far from 30. The average always leans towards whichever isotope there is more of.
  7. 63.6 · 0.70 x 63 + 0.30 x 65 = 44.1 + 19.5 = 63.6.
  8. 80 · Out of 100 atoms: 50 x 79 = 3950 and 50 x 81 = 4050. That is 8000 for 100 atoms, so the answer is 80.
  9. reactions are settled by electrons, and both carry 17 · Her fact is right, since chlorine-37 does weigh more. It just has nothing to do with reacting, because the extra neutrons sit in a nucleus that takes no part.
  10. element A, since neither isotope outweighs the other · An even split puts the average exactly halfway. A 90 to 10 split drags it almost all the way onto the common isotope.
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