Unit 1 – Atomic Theory and Atomic Structure
Section 2 – Atomic Mass, Atomic Number, and Isotopes
1. Consider the Br2 molecule. If there are two common isotopes,
Br atoms are there in Br2? 3 (79 & 79, 79 & 81, 81 & 81)
79Br
and 81Br, how many different combinations of
2. If the two Br isotopes, 79Br and 81Br, each have a natural abundance of about 50%, what is the most likely
molecular weight of Br2 molecules? 160 amu (79 x 0.5) + (81 x 0.5) = 80 x 2 = 160
3. The identity of an element is determined by the number of which particle? protons
4. Hydrogen can occur as 1H (1 amu), 2H (deuterium, 2 amu), and 3H (tritium, 3 amu) with respective natural
abundances of >99.98%, <0.02%, and almost 0, respectively. Do all water molecules have equal masses? Why or
why not? All water molecules do not have equal masses because some may be made with deuterium or tritium. Sometimes called “heavy
water”.
5. Considering only these hydrogen isotopes, how many different masses of water can occur?
1+1+16 = 18
2+3+16 = 21
1+2+16 = 19
3+3+16 = 22
1+3+16 = 20
5
2+2+16 = 20
6. Strontium has four stable isotopes, Strontium-84 has a very low natural abundance, but 86Sr, 87Sr, and 88Sr are
all reasonably abundant. Which of these more abundant isotopes predominates? 88Sr because the atomic mass is over
87.
7. The following diagram is a representation of 20 atoms of a fictitious element which we will call
Stewium (St). The red spheres are 293St and the blue spheres are 295St. Assuming that the
sample here is statistically representative of the element, calculate the percentage abundance
of each isotope. If the mass of 293St is 293.15 amu and the mass of 295St is 295.14 amu, what is
the atomic weight of St?
293
St 
12
 60%
20
295
St 
8
 40%
20
293.15 .6  295.14 .4  293.95 amu
8. Calculate the atomic mass of silicon. The three silicon isotopes have atomic masses and relative abundances of
27.9769 amu (92.2297%), 28.9765 amu (4.6832%) and 29.9738 amu (3.0872%).
27.9769 .922297  28.9765.046832  29.9738.030872  28.0854 amu
9. Copper has two stable isotopes, 63Cu and 65Cu with masses of 62.939598 amu and 64.927793 amu, respectively.
Calculate the percent abundances of these isotopes of copper. You need the atomic mass on the periodic table for this
question.
62.939598  x  64.927793 1  x  63.546
62.939598x  64.927793  64.927793x  63.546
1.988195x  1.381793 x  .6949987
There is 69.500% 63Cu and 30.500% 65Cu
10. Antimony has two naturally occurring isotopes. The mass of antimony-121 is 120.904 amu and the mass of
antimony-123 is 122.904 amu. Using the average mass from the periodic table, find the abundance of each
isotope.
120.904  x  122.904  1  x  121.760
120.904x 122.904 122.904x  121.760
2.000x  1.144 x  0.572
There is 57.2% of antimony-121 and 42.8% antimony 123