Transcript II. Radioactive Decay

CHAPTER 22
Nuclear Chemistry
Types of Radiation
 Isotopes - atoms of the same element with the
same number of protons but different numbers
of neutrons
 Radioisotopes – isotope with an unstable
nucleus that emits radiation to become a more
stable nucleus
 Radioactive Decay – spontaneous reaction in
which unstable nuclei lose energy in the form of
nuclear particles
Nuclear Stability
 Why do atoms decay anyway…
 need stable ratio of neutrons to protons
• Small atoms are stable with a 1 to 1 ratio
of protons to neutrons
• As the atomic number increases, atoms
tend to have more neutrons than protons
with stable ratio increasing to 1 to 1.5
 The type of unbalance that is present in the
nucleus determines the type of decay.
Nuclear Stability
Half-life
 Half-life (t½)
 Time required for half the atoms of a
radioactive nuclide to decay.
 Shorter half-life = less stable.
C. Johannesson
Half-life
mf  m ( )
1 n
i 2
mf: final mass
mi: initial mass
n: # of half-lives
Half-life
 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:
WORK:
t½ = 5.0 s
mi = 25 g
mf = ?
total time = 60.0 s
n = 60.0s ÷ 5.0s
n = 12
mf = mi (½)n
mf = (25 g)(0.5)12
mf = 0.0061 g
Types of Nuclear Particles
Alpha particle (α)
 Composition: 2 protons, 2
neutrons
 Symbol: 4He or α
 Charge: +2
 Penetrating power: low, stopped
by paper or cloth
Beta particle (β)
 Composition: 1 electron
 Symbol: -1 e
 Charge: -1
 Penetrating power: 100 times
greater than alpha, stopped by
wood or concrete
Gamma ray (γ )
 Composition: electromagnetic
waves
 Symbol: γ
 Charge: 0
 Penetrating power: 1000 times
greater than beta, stopped by lead
or 6 feet of concrete
Types of Nuclear Decay
 Alpha Emission
238
92
parent
nuclide
U
Th  He
234
90
daughter
nuclide
4
2
alpha
particle
Numbers must balance!!
Types of Nuclear Decay
 Beta Emission
131
53
I
131
54
Xe  e
0
-1
electron
 Positron Emission
38
19
K
38
18
C. Johannesson
Ar 
0
1
e
positron
Types of Nuclear Decay
 Electron Capture
106
47
Ag  e 
0
-1
106
46
Pd
electron
 Transmutation
 One element becomes another.
Fission vs. fusion
Fission
 splitting a nucleus into two or more smaller
nuclei
 1 g of 235U =
3 tons of coal
235
92
U
Fission
 chain reaction - self-propagating reaction
 critical mass mass required
to sustain a
chain reaction
Fusion
 combining of two nuclei to form one
nucleus of larger mass
 thermonuclear reaction – requires temp of
40,000,000 K to sustain
 1 g of fusion fuel =
20 tons of coal
 occurs naturally in
stars
2
1
H H
3
1
C. Johannesson
Fission vs. Fusion
F
I
S
S
I
O
N
 235U is limited
 danger of meltdown
 toxic waste
 thermal pollution
F
U
S
I
O
N




fuel is abundant
no danger of meltdown
no toxic waste
not yet sustainable