CHAPTER 4
The Structure Of The Atom
1. Use the periodic table to complete the following table.
6. How many more neutrons does thorium-230 have
than protons? How many electrons does thorium-230
have?
7. Show that the mass number and the number of
protons are conserved in the following nuclear
230
4
equation: 234
92 U  90 Th  2 He.
8. Give the mass number of each isotope.
a. Be with 5 neutrons
b. Ga with 39 neutrons
c. Si with 16 neutrons
d. Ti with 26 neutrons
2. Give the number of protons, electrons, and neutrons
in each of the following atoms.
a.
108
47
b.
40
20
Ca
c.
23
11
Na
Ag
9. Give the atomic number of each isotope.
a. magnesium-25
b. bromine-79
c. antimony-121
3. Name each isotope, and write it in symbolic notation.
a. atomic number 26; mass number 56
b. atomic number 29; mass number 64
c. atomic number 17; mass number 37
10. Neon has two isotopes: neon-10 and neon-12.
a. Which isotope has the greater mass?
b. Which has more neutrons?
c. Which has more protons?
d. Which has more electrons?
4. How many protons, electrons, and neutrons are in
each of the following isotopes?
a. uranium-235
b. hydrogen-3
c. silicon-29
11. Use the table below to calculate the atomic mass of
element X. Then use the periodic table to identify the
element. Show all your work.
5. How many neutrons does europium-151 have? What
is the isotope’s mass number?
Chemistry: Matter and Change
1
Supplemental Problems
Chapter 4 (continued)
12. Magnesium has three isotopes. Magnesium-24 has a
percent abundance of 78.99%. Magnesium-26 has a
percent abundance of 11.01%. What is the percent
abundance of magnesium-25? Assume that there are
no other magnesium isotopes.
15. An element has three naturally occurring isotopes.
Information about each isotope is summarized below.
13. Calculate the atomic mass of iridium. Iridium has two
isotopes. Iridium-191 has a mass of 191.0 amu and a
percent abundance of 37.58%. Iridium-193 has a mass
of 193.0 amu and a percent abundance of 62.42%.
Show all your work.
a. Find the atomic mass of this element. Show all
your work.
b. Identify the element, using the periodic table.
c. Write each isotope in symbolic notation.
14. An element has three naturally occurring isotopes.
Isotope 1 has a mass of 19.992 amu.
Isotope 2 has a mass of 20.994 amu.
Isotope 3 has a mass of 21.991 amu.
16. The isotope carbon-14 can be used to determine the
ages of objects that were once living, such as wood,
bones, and fossils. While alive, living things take in
all the isotopes of carbon, including carbon-14.
Carbon-14 undergoes radioactive decay continuously.
After an organism dies, the carbon-14 in its body
continues to decay. However, its body no longer takes
in new carbon-14. Thus, by measuring how much
carbon-14 a once-living object contains and
comparing it with the amount of carbon-14 in a
currently living thing, you can determine the age of
the object.
a. In terms of subatomic structure, how does carbon14 differ from carbon-12 and
carbon-13?
b. How is carbon-14 like carbon-12 and carbon-13?
c. Carbon-14 emits a beta particle as it decays. What
atom does carbon-14 decay to?
d. Write an equation to represent the decay of
carbon-14.
The pie graph shows the relative abundance of each
isotope.
a. Calculate the atomic mass of the element.
b. Identify the element, using the periodic table.
Chemistry: Matter and Change
2
Supplemental Problems
Answer Key
b. atomic number 29; mass number 64
Chapter 4
1. Use the periodic table to complete the following table.
copper-64;
64
29
Cu
Atomic number  number of protons  number of electrons
c. atomic number 17; mass number 37
chlorine-37;
37
17
Cl
4. How many protons, electrons, and neutrons are in
each of the following isotopes?
Atomic number  number of protons  number of
electrons
Number of neutrons  mass number  atomic number
a. uranium-235
2. Give the number of protons, electrons, and neutrons
in each of the following atoms.
92 protons, 92 electrons, 143 neutrons
(23592  143)
Atomic number  number of protons  number of
electrons
b. hydrogen-3
Number of neutrons  mass number  atomic number
a.
108
47
1 proton, 1 electron, 2 neutrons (3 1  2)
Ag
c. silicon-29
47 protons, 47 electrons, 61 neutrons
(10847  61)
b.
40
20
14 protons, 14 electrons, 15 neutrons
(2914  15)
Ca
5. How many neutrons does europium-151 have? What
is the isotope’s mass number?
20 protons, 20 electrons, 20 neutrons
(4020  20)
c.
23
11
Number of neutrons  mass number  atomic number
Na
 151  63  88 neutrons
11 protons, 11 electrons, 12 neutrons
(2311  12)
The mass number is 151.
6. How many more neutrons does thorium-230 have
than protons? How many electrons does thorium-230
have?
3. Name each isotope, and write it in symbolic notation.
a. atomic number 26; mass number 56
iron-56;
56
26
Fe
Chemistry: Matter and Change
3
Supplemental Problems
Answer Key (continued)
Number of neutrons = mass number – atomic number
c. antimony-121
 230  90  140 neutrons
51
Difference between the number of protons and the number
of neutrons  140  90  50
9. Neon has two isotopes: neon-10 and neon-12.
a. Which isotope has the greater mass?
Therefore, thorium-230 has 50 more neutrons than it does
protons.
Atomic number  number of protons  number of electrons
neon-12
Therefore, thorium-230 has 90 electrons.
b. Which has more neutrons?
7. Give the mass number of each isotope.
neon-12
Number of neutrons  number of protons  mass number
c. Which has more protons?
a. Be with 5 neutrons
They have an equal number of protons.
5 neutrons4 protons  9
d. Which has more electrons?
b. Ga with 39 neutrons
They have an equal number of electrons.
39 neutrons31 protons  70
10. Use the table below to calculate the atomic mass of
element X. Then use the periodic table to identify the
element. Show all your work.
c. Si with 16 neutrons
16 neutrons14 protons  30
d. Ti with 26 neutrons
26 neutrons22 protons  48
8. Give the atomic number of each isotope.
From the periodic table,
a. magnesium-25
Mass contribution  (mass)(percent abundance)
X: (15.995 amu)(99.762%)  15.957 amu
16
12
X: (16.999 amu)(0.038%)  0.0065 amu
17
X: (17.999 amu)(0.20%)  0.036 amu
18
b. bromine-79
Atomic mass of X  15.957 amu  0.0065 amu  0.036
amu  16.000 amu
35
The element is oxygen.
11. Magnesium has three isotopes. Magnesium-24 has a
percent abundance of 78.99%. Magnesium-26 has a
percent abundance of 11.01%. What is the percent
abundance of magnesium-25? Assume that there are
no other magnesium isotopes.
Chemistry: Matter and Change
4
Supplemental Problems
Answer Key (continued)
a. Calculate the atomic mass of the element.
All the percentages should add up to 100%.
Therefore:
Mass contribution  (mass)(percent abundance)
Isotope 1: (19.992 amu)(90.51%)  18.09 amu
Isotope 2: (20.994 amu)(0.27%)  0.057 amu
Isotope 3: (21.991 amu)(9.22%)  2.03 amu
Atomic mass  18.10 amu0.057 amu
2.03 amu  0.18 amu
100%  (percent abundance of magnesium-24)(percent
abundance of magnesium-25)(percent abundance of
magnesium-26)
100%  78.99%(percent abundance of magnesium25)11.01%
Percent abundance of magnesium-25 
100%(78.99%11.01%)  10.00%
b. Identify the element, using the periodic table.
12. Calculate the atomic mass of iridium. Iridium has two
isotopes. Iridium-191 has a mass of 191.0 amu and a
percent abundance of 37.58%. Iridium-193 has a mass
of 193.0 amu and a percent abundance of 62.42%.
Show all your work.
The element is neon.
14. An element has three naturally occurring isotopes.
Information about each isotope is summarized below.
Mass contribution  (mass)(percent abundance)
Ir-191: (191.0 amu)(37.58%)  71.78 amu
Ir-193: (193.0 amu)(62.42%)  120.5 amu
Atomic mass of Ir  71.78 amu  120.5 amu
 192.3 amu
13. An element has three naturally occurring isotopes.
Isotope 1 has a mass of 19.992 amu.
Isotope 2 has a mass of 20.994 amu.
Isotope 3 has a mass of 21.991 amu.
a. Find the atomic mass of this element. Show all
your work.
Mass contribution  (mass)(percent abundance)
Isotope 1: (23.985 amu)(78.70%) 
18.88 amu
The pie graph shows the relative abundance of each
isotope.
Isotope 2: (24.946 amu)(10.13%) 
2.531 amu
Isotope 3: (25.983 amu)(11.17%) 
2.902 amu
Atomic mass of element  18.88 amu
2.527 amu2.902 amu  24.31 amu
b. Identify the element, using the periodic table.
The element is magnesium.
c. Write each isotope in symbolic notation.
24
12
Chemistry: Matter and Change
5
Mg ,
25
12
Mg ,
26
12
Mg
Supplemental Problems