CHEM 163 EXPERIMENT
NUCLEAR CHEMISTRY
Name_________________
Isotopes and Radioactivity
After completing this learning activity packet, you should be able to do the following:
(1) Define what isotopes are and give examples.
(2) Write standard isotope symbol for an atom when the numbers of proton, neutron and electron are given; tell
the numbers of proton, neutron and electron present in an atom, when given its standard isotope symbol.
(3) Name and describe three types of natural radioactivity in terms of standard isotope symbol, mass, charge and
penetration power.
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I.
The Atom and Its Nucleus
This description of an atom is based on the nuclear atomic model, first proposed in 1911 by Ernest Rutherford, a
New Zealander doing research in England.
Rutherford developed the nuclear atomic model from the results of his famous gold-foil experiment. He
bombarded a piece of gold foil with small, positively charged alpha particles and traced these particles. As shown
in Figure 1, he found that, as expected, most of the particles shot straight through the foil. However, to his
Figure 1. Rutherford's Gold-Foil Experiment
a bounceback particle
a deflected
particle
stream of fast moving
alpha particles
Gold foil, a few
layers of atoms
thick.
Close-up view of one gold
atom; size of the nucleus
greatly exaggerated.
amazement, a small percentage of the particles deflected to the sides, and a few of them even bounced back!
He concluded that the deflected alpha particles must have encountered positively charged centers in the gold foil,
which deflect them from a straight course due to like-charge repulsion. A few alpha particles may have collided
head-on with these centers, which must be dense or massive enough to bounce the alpha particles backward.
Based on the small ratio of particles affected by the nuclei, he estimated the ratio between the radii of the nucleus
and the atom as being 1: 100,000.
Further experimental evidences to-date confirm Rutherford's idea. In addition, it has been determined that a
proton and a neutron weigh approximately the same, and each of them weighs 1,837 times more than an electron.
The mass of a proton is defined as 1 atomic mass unit, or 1 a.m.u. The mass of an electron is therefore: 1  1,837
= 0.000544 a.m.u. In addition, a proton carries +1 charge, an electron carries -1 charge, and a neutron has no
charge.
Answer the following questions concerning the atom and its nucleus.
Q1. A gold atom contains 79 protons, 118 neutrons and 79 electrons.
(a) What is the overall electrical charge of a gold atom? _________ Explain or show calculation.
_______________________________________________________________________________________
(b) What is the electrical charge of the nucleus of a gold atom? _______ Explain or show calculation.
_______________________________________________________________________________________
(c) What is the mass of a gold atom in terms of a.m.u.? ________ Explain or show calculation.
_______________________________________________________________________________________
(d) What is the mass of the nucleus of a gold atom in terms of a.m.u.? ______ Explain or show calculation.
_______________________________________________________________________________________
Q2. An alpha particle consists of 2 protons, 2 neutrons and no electron.
(a) What is the overall electrical charge of an alpha particle? ________
Explain or show calculation.
_______________________________________________________________________________________
(b) What is the mass of an alpha particle in terms of a.m.u.? ________
Explain or show calculation.
_______________________________________________________________________________________
(c) Compare the mass of the gold nucleus with that of the alpha particle. Explain why it is reasonable to expect
that an alpha particle will bounce back from a gold nucleus in a head-on collision of the two, as observed in the
gold-foil experiment.
_______________________________________________________________________________________
II. Different Types of Atoms
A. Isotopes
The number of protons is unique in all the atoms of the same element. It is defined as the atomic number of the
element, and is given for each element in the periodic table. The atoms of the same element, however, may have
different numbers of neutrons. Isotopes are atoms of the same element with different numbers of neutrons.
Q3. (a) How are the isotopes of an element similar to each other?__________________________________
(b) How do the isotopes of an element differ?__________________________________________________
B.
Mass Numbers and Averaged Atomic Mass
The sum of the protons and the neutrons in an atom is defined as the mass number. This number is unique for
each isotope of an element. An isotope is often identified by the symbol of an element followed by a specific
mass number. For example, Ne-20 denotes an isotope with mass number = 20 or Ne-22 denotes another isotope
with mass number = 22. Mass number approximately equals the mass of the atom, because each proton and
neutron weighs approximately 1 a.m.u., and the mass of electrons is negligible.
Q4. Calculate the mass numbers of the three isotopes of hydrogen from lecture: _____, ______ and _______
Q5. What is the mass number of the most abundant isotope of carbon? ____________
Q6. Co-60 identifies an isotope of the element named ______________ , 60 denotes ____________.
C. Standard Isotope Symbols
The notation as Si-30 or Silicon-30 identifies an isotope, but more information concerning the atom is provided
by the standard isotope symbol. The standard isotope symbol specifies in a concise way all the following
information of an atom: its elemental symbol, mass number, atomic number and the electrical charge. Each
information is positioned in a particular location in the isotope symbol, as shown in Figure 1, next page.
mass number
(upper left corner)
A
atomic number
(lower left corner)
Z
element
symbol
(main body)
net charge (upper right
corner, not indicated if
the net charge is zero)
# neutrons
(lower right corner)
Figure 1. Standard Isotope
Symbol
The standard isotope symbols are: 11 H 01 and 42 He 2 2 . Answer the following questions
Q7. (a) In the symbol 11 H 01 , the atomic number is ________ , (b) the mass number is ________ and
(c) the net charge of this isotope is ______ . The symbol 42 He 2 2 , indicates the isotope's atomic number is
(d) ______, its mass number is (e) _____, and the net charge is (f) _______ .
Q8. Calculate the numbers of subatomic particles present from isotope symbols. Remember that:
Number of protons = Atomic Number
Number of neutrons = Mass number  atomic number
Number of electrons = Atomic number - net charge
(a) The symbol, 11 H 01 denotes a hydrogen isotope with: ______ proton, ______ neutron, and _______ electron.
(b) The symbol 42 He 2 2 denotes: element name ____________ with _____ p+, ______ n0 and ________ e-.
Q9. Complete Table 1 with appropriate isotope symbols and numbers of subatomic particles.
Table 1. Standard Isotope Symbols
Isotope Symbol
number of
number of
number of
protons
neutrons
electrons
2
2
2
30
26
mass
number
atomic
number
26
197 Au
79 118
60 Co
27 33
40 Ca 2
20 20
30
Isotope Symbol
number of
proton
number of
neutron
28
65
numbr of
electron
mass
number
atomic
number
16 O 2
8 8
78
54
53
35 Cl1
17 18
III. LAB EXCERISE
Nuclear Changes and Radioactivity
A nuclear change involves changes in the nucleus of the atom. A nuclear change will involve often with the lost
or gain of protons and /or neutrons. An atom of one element may turn into that of another element with a different
atomic number, and/or mass number.
There are three types of natural radiation, named after the first three letters of the Greek alphabet: alpha (), beta
() and gamma (). These can be detected by a Geiger radiation counter.
In general terms, a Geiger tube collects the energetic particles or rays of radiation, and these generate electrical
impulses in the tube. The electrical impulses are noted by a counter and reported with a digital display, a meter, or
as audible clicks.
Procedures:
1. Connect a radiation monitor to DIG channel port on the LabQuest and choose NEW from the File menue.
a. See instructor if Monitor does not appear in LabQuest screen
2. Set up data-collection mode
a. On screen, tap Mode. Change mode to Events with Entry
b. Enter the Name (Reading) and leave the Units field blank. A value of 50 seconds/interval is acceptable
c. Select OK
3. Set up the monitor
a. Place the source near the metal screen. To test an absorber, place absorber between the source and the screen
b. Use approximately the same position for the sources each time, with and without an absorber. The plastic disc
sources emit the most radiation from the underside.
4. Determine background count rate
a. Move all sources away from the monitor
b. Start data collection to prepare the system to collect data
c. Tap Keep. There will ba a 50 sec delay for counting
d. Enter 0 as the Reading value to indicate for background count. Select OK to store data pair.
5. Test beta source
a. Place beta cource near monitor, as directed in Step 3
b. Tap Keep. Counts are taken for 50 sec
c. Enter 1 as the Reading value. Select OK
d. Place a single sheet of paper between the beta source and the monitor
e. Select Keep. When the count is complete, enter 2 as Reading value, select OK
f. Repeat process using aluminum as the shield. Select Keep, enter 3 as Reading value, select OK
g. Stop data collection
6. Record the data in the data table
7. Start data collection and repeat process with the Gamma source
DATA TABLE
Background (cpm)
1st count
No Shielding
Background
Beta source
Gamma source
Alpha source
2nd count
Paper Sheild
3rd count
Al Shield
Shield
average cpm
Shield
Shield
Q10. Complete the following table concerning the three types of natural radiation you studied.
Table 3. Compositions of Three Types of Natural Radiation
standard isotope symbol
4
2
2
4 2
He 2 or 2  2
0
1
1
e or
0
1

1
0
0
0
0
name of radiation
number of proton(s) present
number of neutron(s) present
number of electron(s) present
Q11. Answer the following questions. You may not have be able to collect data for the Alpha sourc
(a) Which radiation, alpha, beta or gamma, has highest penetration power (not cut off by the shields)? ____
(b) Which radiation, alpha, beta or gamma, has the highest mass?__________________________
(c) Which radiation, alpha, beta or gamma, carries a negative electrical charge?___________________
(d) Which radiation, alpha, beta or gamma, is cut off completely by the lead shield? __________________
FURTHER STUDIES Isotope Symbols
Q12. Write standard isotope symbols for the following ions or isotopes:
(a) barium with 54 electrons & 81 neutrons
(b) oxygen with -2 charge and 10 neutrons
(c) lithium with 4 neutrons
(d) An ion with 2p+1 and 2n0, zero e-1
(e) An ion with 35p+1, 45n0 and 36e-1
(f) An ion with 56 p+1, 81n0 and 54e-1
Q13. (a) Complete the following table.
standard isotope
symbol
# proton
# neutron
# electron
12
13
12
26
26
mass
number
atomic
number
magnesium-25
58
iron-58
65 Cu  2
29 36
standard isotope
symbol
name
copper-65
# proton
# neutron
# electron
mass
number
atomic
number
92
235
92
name
14 C
6 8
37 Cl
17 20
17
18
18
(b) Which of the above are isotopes to each other?_____________________ and __________________.
Averaged Atomic Weight
Isotopes are atoms of the same element with different numbers of neutrons.
For example, chlorine has two major isotopes: 75% is chlorine-35 with 18 neutrons, and 25% is chlorine-37 with
20 neutrons. (These percentages are called % natural abundance.) Chlorine-35 has the atomic mass of 35 a.m.u.
and chlorine-37 has the atomic mass of 37 a.m.u. The mass reported for chlorine in the periodic table is neither 35
nor 37; it is a weighted average of these two numbers. The weighted average is calculated by taking 75% of mass
35 plus 25% of mass 37:
(35 amu x 75%) + (37 amu x 25%) = (35 x 0. 75) + (37 x 0. 25) = 35. 5 amu
Q14. Answer the following questions.
(a) mass of chlorine on the Periodic Table: ______.
(b) Is it close to 35. 5? _____
(c) Try a similar calculation for another element, copper. Given the following information:
Copper has two major isotopes: 69.1% of copper-63 and 30.9% of copper-65.
( ________ amu  ________%) + ( ________ amu  _______%)
= (_______  ________) + (________  _________)
= _______________ amu. How does this value compare to the reported mass in the table?___________
Natural Abundance of Different Isotopes
Q15.
Radioactivity is of two types, Natural and Induced radioactivity. Define each type.
a. Natural radioactivity: ______________________________________________________________
b. Induced radioactivity: ______________________________________________________________
c. A stable nucleus may be coverted to an unstable nucleus by _______________________ with highenergy particles or radiation.
d. New elements have been made by bombarding nuclei of heavy elements with nuclei of light elements.
Elements with atomic number greater than 92, are made this way. What is the term for these elements?
______________________________________________
Isotopes are atoms of the same element with different neutron numbers. Almost every element has two or more
isotopes in existence naturally. An element exists in nature with a certain percentage of its atoms being of one
isotope, and other percentages being of other isotopes. This percentage break-down of isotopes for each element
has been compiled and tabulated in the CRC Handbook of Chemistry and Physics.
Obtain a copy of the CRC Handbook of Chemistry and Physics. In the index, look up the entry: “Isotopes abundance, % natural abundance table”, and locate the complete table listing this information. You will see a
detailed table of isotopes listed in order of atomic number. Acquaint yourself with the natural isotopes of the
following common elements with the information given in this table.
Q16. Complete the following.
(a) The CRC Handbook I used is of the _______ edition, published in the year _____________ .
(b) How many different isotopes of oxygen are listed in this Handbook? _________
(c) Which isotope of oxygen has the highest percentage of natural abundance? ____________
Note: natural abundance refers to the natural occurrence of an isotope in nature. The isotope with highest
natural abundance is the most common or predominant isotope of that element.
Which is the most common natural isotope of nitrogen? _______ What is its % natural abundance? ___
(d) The isotope of hydrogen H-1, is the most common in nature. Its % natural abundance is ____________ .
(e) The isotope H-2 (named deuterium), is rare in nature. Its % natural abundance is ____________ .
Questions For Review what you have learned: Isotopes and Radioactivity
KEY TERMS: you have read or used the following important terms or concepts. Write a sentence or two to
describe each of them in your own words first.
1. (a) What are the three types of natural radiation? ___________________________________________
(b) Describe each one in terms of mass, charge and penetration power.________________________________
____________________________________________________________________________________________________
2. What is background radiation? ________________________________________________________________
____________________________________________________________________________________________________
3. Describe what is meant by Nuclear Change? _____________________________________________________
___________________________________________________________________________________________
Explain, analyze, apply, evulate what you have learned.
4. What information can be obtained from a standard isotope symbol? __________________________________
___________________________________________________________________________________________
5. How does the gold foil experiment provide evidences for the nuclear model of the atom?__________________
___________________________________________________________________________________________
6. (a) Is it true that one can avoid alpha radiation by wearing gloves and regular clothing? ______
(b) Explain your answer to (a)? _______________________________________________________________
____________________________________________________________________________________________________
7. (a) Is it true that one must use a lead shield to avoid beta radiation?__________
(b) What is the reasoning for your answer? ______________________________________________________
Absorber
Dector
Radiation Source
Cable
Plug
Counter screen
Distance marked that
dector is from the source
absorders