Transcript Section 24-2

Section 24.2 Radioactive Decay
• Explain why certain
nuclei are radioactive.
• Apply your knowledge
of radioactive decay to
write balanced nuclear
equations.
• Solve problems
involving radioactive
decay rates.
radioactivity: the
process by which some
substances
spontaneously emit
radiation
Section 24.2 Radioactive Decay (cont.)
transmutation
positron
nucleon
electron capture
strong nuclear force
radioactive decay series
band of stability
half-life
positron emission
radiochemical dating
Unstable nuclei can break apart
spontaneously, changing the identity of
atoms.
Nuclear Stability
• Except for gamma radiation, radioactive
decay involves transmutation, or the
conversion of an element into another
element.
• Protons and neutrons are referred to as
nucleons.
• All nucleons remain in the dense nucleus
because of the strong nuclear force.
Nuclear Stability (cont.)
• The strong nuclear force acts on
subatomic particles that are extremely
close together and overcomes the
electrostatic repulsion among protons.
Nuclear Stability (cont.)
• As atomic number increases, more and
more neutrons are needed to produce a
strong nuclear force that is sufficient to
balance the electrostatic repulsion between
protons.
• Neutron to proton ratio increases gradually to
about 1.5:1.
Nuclear Stability (cont.)
• The area on the graph
within which all stable
nuclei are found is known
as the band of stability.
• All radioactive nuclei are
found outside the band.
• The band ends at
Pb-208; all elements with
atomic numbers greater
than 82 are radioactive.
Types of Radioactive Decay
• Atoms can undergo different types of
decay—beta decay, alpha decay, positron
emission, or electron captures—to gain
stability.
Types of Radioactive Decay (cont.)
• In beta decay, radioisotopes above the
band of stability have too many neutrons to
be stable.
• Beta decay decreases the number of
neutrons in the nucleus by converting one to
a proton and emitting a beta particle.
Types of Radioactive Decay (cont.)
• In alpha decay, nuclei with more than 82
protons are radioactive and decay
spontaneously.
• Both neutrons and protons must be reduced.
• Emitting alpha particles reduces both
neutrons and protons.
Types of Radioactive Decay (cont.)
Types of Radioactive Decay (cont.)
• Nuclei with low neutron to proton ratios
have two common decay processes.
• Positron emission is a radioactive decay
process that involves the emission of a
positron from the nucleus.
• A positron is a particle with the same mass
as an electron but opposite charge.
• Positron (+)
– positron
0
1
e
1+
foil
Types of Radioactive Decay (cont.)
• During positron emission, a proton in the
nucleus is converted to a neutron and a
positron, and the positron is then emitted.
• Electron capture occurs when the nucleus of
an atom draws in a surrounding electron and
combines with a proton to form a neutron.
Types of Radioactive Decay (cont.)
Types of Radioactive Decay (cont.)
Writing and Balancing Nuclear Equations
• Nuclear reactions are expressed by
balanced nuclear equations.
• In balanced nuclear equations, mass
numbers and charges are conserved.
• Alpha Emission
238
92
parent
nuclide
U
Th  He
234
90
daughter
nuclide
4
2
alpha
particle
Numbers must balance!!
Section 24-2
• Beta Emission
131
53
I
131
54
Xe  e
0
-1
electron
• Positron Emission
38
19
K  Ar  e
38
18
0
1
positron
• Electron Capture
106
47
Ag  e 
0
-1
106
46
electron
Pd
Radioactive Decay Rates
• A half-life is the time required for one-half of
a radioisotope to decay into its products.
N is the remaining amount.
N0 is the initial amount.
n is the number of half-lives that have passed.
t is the elapsed time and T is the duration of the
half-life.
mf  m ( )
1 n
i 2
mf: final mass
mi: initial mass
n: # of half-lives
• Fluorine-21 has a half-life of 5.0
seconds. If you start with 25 g of
fluorine-21, how many grams would
remain after 60.0 s?
GIVEN:
t½ = 5.0 s
mi = 25 g
mf = ?
total time = 60.0 s
n = 60.0s ÷ 5.0s =12
WORK:
mf = mi (½)n
mf = (25 g)(0.5)12
mf = 0.0061 g
Radioactive Decay Rates (cont.)
• A radioactive isotope has a beginning
mass of 50.00 grams. After 2.000 hours
only 3.125 grams are left. Determine the
half-life (in minutes) of the isotope.
Section 24.2 Assessment
The process of converting one element
into another by radioactive decay is
called ____.
A. half-life
A
0%
D
D. trans-decay
C
C. transmutation
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. nuclear conversion
Section 24.2 Assessment
An unknown element has a half-life of 40
years. How much of a 20.0g sample will be
left after 120 years?
A. 0.00g
A
0%
D
D. 7.50g
C
C. 5.00g
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. 2.50g