Isotopes & Average Atomic Mass — Drills · Grade 11 Science

Chapter: Atoms, Electrons & Periodicity

What these drills covers

By the end of Isotopes & Average Atomic Mass you should be able to:

6 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: isotopes, percent abundance, fractional abundance, average atomic mass, weighted average, mass spectrometer.

Problem 1

Two isotopes of the same element differ only in their number of…
  1. orbitals
  2. protons
  3. electrons
  4. neutrons
Answer and explanation

neutrons

Isotopes have the same protons (same element) but different numbers of neutrons.

Problem 2

Carbon has two main isotopes: carbon-12 (12.00 amu, 99.0%) and carbon-13 (13.00 amu, 1.0%). Find carbon's average atomic mass in amu.
Answer and explanation

12.01 amu

12.00 × 0.990 + 13.00 × 0.010 = 11.88 + 0.13 = 12.01 amu.

Problem 3

An element has isotopes of mass 35.0 amu (75.0%) and 37.0 amu (25.0%). Find its average atomic mass in amu.
Answer and explanation

35.5 amu

35.0 × 0.750 + 37.0 × 0.250 = 26.25 + 9.25 = 35.5 amu (this is chlorine).

Problem 4

Silver has two isotopes: silver-107 (106.9 amu, 51.8%) and silver-109 (108.9 amu, 48.2%). Find silver's average atomic mass in amu.
Answer and explanation

107.86 amu

106.9 × 0.518 + 108.9 × 0.482 = 55.37 + 52.49 = 107.86 amu.

Problem 5

Why does changing the number of neutrons NOT change which element an atom is?
Answer and explanation

An element's identity is set by its number of protons. Adding or removing neutrons changes only the mass (making a different isotope); the proton count, and therefore the element, stays the same.

Problem 6

An element has two isotopes, of mass 20.0 amu and 22.0 amu, and an average atomic mass of 20.18 amu. What percent of its atoms are the mass-22 isotope?
Answer and explanation

9 %

Let x be the fraction of mass-20: 20x + 22(1 − x) = 20.18 → 22 − 2x = 20.18 → x = 0.91. So the mass-22 fraction is 0.09 = 9%.

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