Transcript Chapter 2 Atoms and Elements

Nuclear Radiation
Natural Radioactivity
A person working with radioisotopes
wears protective clothing and gloves
and stands behind a shield.
1
Radioactive Isotopes
A radioactive isotope
• has an unstable nucleus
• emits radiation to become more stable
• can be one or more isotopes of an element
• is written with a mass number and an atomic
number
• includes the mass number in its name
Example: iodine-131
2
Examples of Radioactive
Isotopes
3
Learning Check #1
Thallium-201 is used for heart scans to determine
cardiac function.
A. How many protons are in thallium-201?
B. How many neutrons are in thallium-201?
C. What is the atomic symbol of thallium-201?
4
Nuclear Radiation
Nuclear radiation
• is the radiation emitted by an unstable atom
• takes the form of alpha particles, neutrons,
beta particles, positrons, or gamma rays
5
Alpha Particle
An alpha () particle has
• a helium nucleus
• 2 protons and 2 neutrons
• a mass number of 4
• a charge of 2+
• a low energy compared to
other radiation particles
6
Beta Particle
A beta () particle
• is a high-energy electron
• has a mass number of 0
• has a charge of 1• forms in an unstable nucleus when a neutron
changes into a proton and an electron
7
Positron
A positron ( +)
• has a mass number of 0
• has a charge of 1+
• forms in an unstable nucleus when a proton
changes into a neutron and a positron
8
Gamma () Ray
A gamma () ray
• is high-energy radiation
• has a mass number of 0
• has a charge of 0
• is emitted from an unstable nucleus to give a
more stable, lower energy nucleus
9
Summary of Common Forms of
Nuclear Radiation
10
Learning Check #2
Give the mass number and charge of each type of
radiation.
1. alpha particle
2. positron
3. beta particle
4. neutron
5. gamma ray
11
Radiation Protection
Radiation protection requires
• paper and clothing for alpha particles
• a lab coat or gloves for beta particles
• a lead shield or a thick concrete wall for gamma
rays
• limiting the amount of time spent near a
radioactive source
• increasing the distance from the source
12
Radiation and Shielding
Required
13
Radiation Protection
Different types of shielding are needed for
different radiation particles.
14
Learning Check #3
Indicate the type of radiation
(alpha, beta, and/or gamma)
protection for each type of
shielding.
1) heavy clothing
2) paper
3) lead
4) lab coat
5) thick concrete
15
A person working with radioisotopes
wears protective clothing and gloves
and stands behind a shield.
Nuclear Radiation
Nuclear Equations
A radon gas detector is used to
determine radon levels in buildings
with inadequate ventilation.
16
Radioactive Decay
In radioactive decay,
• a nucleus spontaneously emits radiation
• a nuclear equation is written for the radioactive
nucleus, the new nucleus, and the radiation
emitted
Radioactive nucleus
new nucleus + radiation (, , , +)
17
Nuclear Equations
In a nuclear equation, the type of radiation
emitted causes
• changes in the mass numbers
• changes in the atomic numbers for the nuclei
of the unstable and stable nuclei
18
Balancing Nuclear Equations
In a balanced nuclear equation,
• the sum of the atomic numbers for the nuclei
of the reactant and the products must be
equal
Total =
Mass Numbers
238
=
238
238
92
Total =
19
U

234
90
Th +
92
=
92
Atomic Numbers
4
2
He
Guide to Completing a Nuclear
Equation
20
Alpha Decay
In alpha decay, a
radioactive nucleus emits
an alpha particle to form a
new nucleus that has
• a mass number 4 less
than that of the initial
nucleus
• an atomic number that
has decreased by 2 from
that of the initial nucleus
21
Example of Writing an Equation
for Alpha Decay
Write an equation for the alpha decay of 22286 Rn
STEP 1 Write the incomplete nuclear equation.
222
86
Rn
 ? +
4
2
He
STEP 2 Determine the missing mass number.
222
= ? + 4
222 – 4 = 218
218
22
= ? (mass number of new nucleus)
Equation for Alpha Decay
(continued)
STEP 3 Determine the missing atomic number.
86
= ? + 2
86 – 2 = ?
84
= ? (atomic number of new nucleus
STEP 4 Determine the symbol of the new nucleus.
Symbol of element 84
218
84
= Po
Po
STEP 5 Complete the nuclear equation.
222
86
23
Rn

218
84
Po +
4
2
He
Beta Decay
Beta decay occurs when
• an electron (beta
particle) is emitted
from the nucleus
• a neutron in the
nucleus breaks down
1
0
24
n

0
-1
e +
1
1
H
Writing an Equation for a Beta
Emitter
Write an equation for the beta decay of potassium-42.
STEP 1 Write the incomplete nuclear equation.
42
19
K  ? +
0
-1
e
STEP 2 Determine the missing mass number.
42
= ? +0
42  0 = 42 = ? (mass number of new nucleus)
STEP 3 Determine the missing atomic number.
19
= ?1
19 + 1 = ?
20
= ? (atomic number of new nucleus)
25
Writing an Equation for a Beta
Emitter (continued)
STEP 4 Determine the symbol of the new nucleus.
Symbol of element 20
42
20
= Ca
Ca
STEP 5 Complete the nuclear equation.
42
19
26
K

42
20
Ca
+
0
-1
e
Learning Check #4
Write the nuclear equation for the beta
decay of Xe-133.
27
Positron Emission
In positron emission,
• a proton is converted to a neutron and a positron
1
1
H 
1
0
n

0
+1
e
• the mass number of the new nucleus is the same,
but the atomic number decreases by 1
49
25
28
Mn

49
24
Cr +
0
+1
e
Learning Check #5
Write the nuclear equation for the positron
emission by Rb-82.
29
Gamma ( ) Radiation
In gamma radiation,
• energy is emitted from an unstable nucleus,
indicated by m following the mass number
• the mass number and the atomic number of the
new nucleus are the same
99m
43
30
Tc

99
43
Tc +

0
0
Summary of Types of Radiation
31
Producing Radioactive Isotopes
Radioactive isotopes are produced
• when a stable nucleus is converted to a
radioactive nucleus by bombarding it with a
small particle
• in a process called transmutation
32
Learning Check #6
What radioactive isotope is produced when a
neutron bombards cobalt-59 and an alpha particle is
emitted?
59
27
33
Co +
1
0
n
 ? +
4
2
He
Nuclear Radiation
Radiation Measurement
A radiation technician uses a Geiger
Counter to check radiation levels.
34
Radiation Detection
A Geiger counter
• detects beta and gamma radiation
• uses ions produced by radiation to create
an electrical current
35
Radiation Measurement
Radiation units of activity include
• curie (Ci)
SI unit = becquerel (Bq)
number of atoms that decay in one second
1 Ci = 3.7 x 1010 disintegrations/s
1 Ci = 3.7 x 1010 Bq
• rad (radiation absorbed dose) SI unit = gray(Gy)
radiation absorbed by the tissues of the body
1 Gy = 100 rad
• rem (radiation equivalent) SI unit = sievert (Sv)
biological damage caused by different types of
radiation = absorbed dose (rad) x factor
1 Sv = 100 rem
36
Units of Radiation
Measurement
37
Exposure to Radiation
Exposure to radiation
occurs from
• naturally occurring
radioisotopes
• medical and dental
procedures
• air travel, radon, and
smoking cigarettes
38
Radiation Sickness
Radiation sickness
• depends on the dose of radiation received at
one time
• is not detected under 25 rem
• involves a decrease in white blood cells at
100 rem
• includes nausea and fatigue over 100 rem
• reduces white-cell count to zero over 300 rem
• leads to death in 50% of people at 500 rem
39
Nuclear Radiation
Half-Life of a Radioisotope
The age of the Dead Sea scrolls was
determined using carbon-14.
40
Half-Life
The half-life of a radioisotope is the time for
the radiation level (activity) to decrease (decay)
to one-half of its original value.
41
Decay Curve
A decay curve shows
• the decay of radioactive atoms
• the remaining radioactive sample
42
Half-lives of Some Radioisotopes
Half-lives of radioisotopes that are
• naturally occurring tend to be long
• used in nuclear medicine tend to be short
43
Half-Life Calculations
•
•
In one half-life, 40 mg of a radioisotope decays to
20 mg.
After two half-lives, 10 mg of radioisotope remain.
40 mg x 1 x 1 = 10 mg
2
2
Initial
40 mg
1 half-life
2 half-lives
20 mg
10 mg
44
Examples of Half-Lives
45
Using Half-lives
46
Learning Check #7
The half-life of iodine-123 is 13 h. How much of a
64-mg sample of I-23 is left after 26 h?
1) 32 mg
2) 16 mg
3) 8 mg
47
Nuclear Radiation
Medical Applications Using Radioactivity
48
Medical Applications
Radioisotopes with short half-lives are used in
nuclear medicine because they
• have the same chemistry in the body as the
nonradioactive atoms
• give off radiation that exposes a photographic
plate (scan), giving an image of an organ
49
Using a Scanner to Detect
Radiation
(a) A scanner is used to detect
radiation from a radioisotope that has
accumulated in an organ. (b) A scan of
the thyroid show the accumulation of
radioactive iodine-131 in the thyroid.
50
Radioisotopes in Nuclear
Medicine
51
Learning Check #8
Which of the following radioisotopes are most likely
to be used in nuclear medicine?
1) K-40 half-life 1.3 x 109 years
2) K-42 half-life 12 hours
3) I-131 half-life 8 days
52
Positron Emission Tomography
(PET)
In positron Emission Tomography (PET),
• positrons are emitted from positron emitters such
as carbon -11, oxygen-15, and fluorine-18
F  188 O + 10 e
positrons are used to study brain function and
metabolism
positrons combine with electrons to produce
gamma ray that can be detected, giving a threedimensional image
18
9
•
•
0
1
53
e +
0
1
e


0
0
Positron Emission Tomography
(PET)
These PET scans of the brain show a normal brain on
the left and a brain affected by Alzheimer’s disease on
the right.
54
Computed Tomography (CT)
In computed tomography (CT), 30 000 X-rays
• are directed at the brain in layers
• are absorbed at different rates due to differences
in densities of body tissues and fluids
• create three-dimensional images of the brain and
any tumors or hemorrhages
55
Computed Tomography (CT)
A CT scan shows a brain tumor (yellow)
on the right side of the brain.
56
Magnetic Resonance Imaging
(MRI)
In magnetic resonance imaging (MRI)
• protons in the presence of a strong magnetic field
align with the magnetic field
• protons release energy when magnetic field is
removed
• protons in different environments emit energies of
different frequencies to provide images of soft
tissue
57
Magnetic Resonance Imaging
(MRI)
An MRI scan provides images of the heart
and lungs.
58
Nuclear Radiation
Nuclear Fission and Fusion
59
Nuclear Fission
In nuclear fission,
• a large nucleus is bombarded with a small
particle
• the nucleus splits into smaller nuclei and
several neutrons
• large amounts of energy are released
60
Nuclear Fission
When a neutron bombards U-235,
• an unstable nucleus of U-236 forms and
undergoes fission (splits)
• smaller nuclei are produced such as Kr-91 and
Ba-142
• neutrons are released to bombard more U-235
nuclei
61
Nuclear Fission
1
0
n + 23592 U  236
92 U 
62
91
36
1
Kr + 142
Ba
+
3
56
0 n + energy
Chain Reaction
A chain reaction occurs
• when a critical mass of uranium undergoes
fission
• releasing a large amount of heat and
energy that produce an atomic explosion
63
Chain Reaction
64
Learning Check #9
Supply the missing atomic symbol to complete the
equation for the following nuclear fission reaction.
1
0
n +
65
235
92
U 
137
52
Te + ? + 210 n + energy
Nuclear Power Plants
In nuclear power plants,
• fission is used to produce energy
• control rods in the reactor absorb
neutrons to slow the fission process
Nuclear power plants supply about 10%
of the electricity in the United States.
66
Nuclear Power Plants
67
Nuclear Fusion
Nuclear fusion
• occurs at extremely high temperatures
(100 000 000 °C)
• combines small nuclei into larger nuclei
• releases large amounts of energy
• occurs continuously in the sun and stars
68
Nuclear Fusion
69
Learning Check #10
Indicate if each of the following is
1) nuclear fission or 2) nuclear fusion
___ A. a nucleus splits
___ B. large amounts of energy are released
___ C. small nuclei form larger nuclei
___ D. hydrogen nuclei react
___ E. several neutrons are released
70
Concept Map
71