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Isotopes and Atomic Mass: Flashcards

Revise isotopes, natural abundance and weighted average atomic mass.

9 cardsIntermediate

9 cards

  1. Question
    What does the atomic mass on the periodic table represent?
    Answer

    The weighted average mass of an element's atoms, based on the natural abundance of each isotope.

  2. Question
    What is natural abundance?
    Answer

    The percentage (or fraction) of an element's atoms that are a particular isotope.

  3. Question
    How do you calculate an average atomic mass?
    Answer

    Multiply each isotope's mass by its abundance as a decimal fraction, then add the results.

  4. Question
    Why is chlorine's atomic mass (35.45 u) closer to 35 than to 37?
    Answer

    Chlorine-35 is about three times as common as chlorine-37, so it carries more weight in the average.

  5. Question
    What instrument measures isotope masses and abundances?
    Answer

    A mass spectrometer.

  6. Question
    What should the abundances of all an element's isotopes add up to?
    Answer

    100% (a total fraction of 1).

  7. Question
    Boron is 19.9% boron-10 (10.013 u) and 80.1% boron-11 (11.009 u). What is its average atomic mass?
    Answer

    (0.199×10.013)+(0.801×11.009)=10.81(0.199 \times 10.013) + (0.801 \times 11.009) = 10.81 u

  8. Question
    What is the difference between mass number and isotope mass?
    Answer

    Mass number is a whole-number count of protons + neutrons (e.g. 35); isotope mass is the measured mass (e.g. 34.969 u).

  9. Question
    Why is a simple average of isotope masses usually wrong?
    Answer

    Isotopes are not equally common; each mass must be weighted by its abundance.