Post on 28-Dec-2015
NuclearChemistry
________________ involve changes with electrons.
________________ involve changes in atomic nuclei.
Spontaneously-changing nuclei emit ________ andare said to be _________.
Chemical reactions
Nuclear reactions
radiationradioactive
energy and/or particles
HonorsChemistry
Radioactivity
nucleons:
mass number:
isotopes: species having the same number of p+, but different numbers of n0
-- radioactive ones are called ___________
nuclide: a nucleus w/a specified number of p+ and n0
-- radioactive ones are called ___________
radioisotopes
radionuclides
protons (p+) and neutrons (n0)
(p+ + n0) in a given atom
these are unstableand emit radiation
atomic number: (Z); # of p+
Radioactive Decay
For nuclear equations, mass (top) and charge (bottom)must balance.
alpha (a) decay:
a-particle (i.e., a He nucleus): massive, slow-moving;
stopped by skin
23492 U 230
90 Th + 42 He
(go DOWNtwo #s on Table)
beta (b) decay:
b-particle (i.e., a fast-moving electron): little mass; stops
~1 cm into body
23491 Pa 234
92 U + 0–1 e
(go UPone # on Table)
In b-decay, the effect is that a n0 is convertedinto a p+, ejecting an e– from the nucleus.
10 n 1
1 p+0–1 e
NOTE: There are no e– in the nucleus. The ejected e–
is formed when energy released from the nucleus “congeals” into mass, via _______. E = mc2
gamma radiation:
-- can penetrate to internal organs
-- gamma ray:
emitted when nucleonsrearrange into a morestable configuration
-- gamma radiation often accompanies other nuclear decays
consists of high-energy photons
00g
(or just g)
23492 U 230
90 Th + 42 He + 0
0g2
-- The shell model says that nucleons reside in shells, similar to… how e– reside in shells.
positron decay: 2312 Mg 23
11 Na + 01 e + 0
0n
neutrino: “massless,” chargeless particle
positron: identical to an e–, but (+)
116 C + 0
–1 e + 00n
electron capture: nucleus captures orbiting e–
115 B
The effect of positron decay and electron captureis to turn a p+ into a n0.
10 n1
1 p +01 e
POSITRON DECAY
10 n1
1 p + 0–1 e
ELECTRON CAPTURE
Nuclear Stability
Nucleons are held together by the __________. strong force
At Z > 83, none are stable(i.e., all are radioactive).
0 83
# of n0
Z (i.e., # of p+)
Band (or Belt)of Stability
~1 n0 : 1 p+
~1.5 n0 : 1 p+
n0
Nuclei that… …have too many… …and stabilize by…
…are above belt…
…are below belt…
…have Z > 83…
p+
p+ and n0
b-emission
positron emission
a-emission
(or e– capture)
Examples: 24294 Pu 238
92 U+42 a(a)
16364 Gd 163
65 Tb+0–1 b(b)
14565 Tb 145
64 Gd+01 e
(positron emissionor e– capture)
14565 Tb 145
64 Gd+ 0–1 e
A radioactive series is the decay sequence a radionuclide goes through to become stable.
e.g., U-238 Th-234 Pa-234 , etc. a b
-- there are three basic series, ending with…
Pb-206, Pb-207, and Pb-208.
Stable Isotope
Radioactive materials will continue to decay until they reach a stable material (Usually with an atomic number less than 83.)
Nuclear Transmutations
These are induced by a bombarding particle,and are typically written in the following order:
targetnucleus
bombardingparticle
Ernest Rutherford wasthe first to artificiallytransmute elements.
ejectedparticle
2713 Al 30
15 P+ 42 He + 1
0 n
This reaction is abbreviated…
2713 Al (a, n) 30
15 P
productnucleus
Write the shorthand for
Write the equation for 147 N (a, p) 17
8 O
2713 Al 24
11 Na+ 10 n + 4
2 He
2713 Al (n, a) 24
11 Na
147 N 17
8 O+ 42 He + 1
1 H
147 N 17
8 O+ 42 a + 1
1 p
“Otzi” the Iceman lived circa 3300 B.C.,according to radiocarbon dating analyses.
Rates of Radioactive Decay
Each radioisotope has a unique rate of decay,its half-life, t1/2, which is the time required forhalf of a sample of a radioisotope to decay intosomething stable. An isotope’s half-life is:
(1) independent of T, P, and its state of chemical combination
(2) useful in radioactive dating.
Half-lives can be as short as a fraction of a second or billions of years. Different nuclei have different decay patterns, depending on why they are unstable (i.e. too many protons, too many neutrons, )
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.
In general, the further away an isotope is from the “band,” the shorter its half life.
= 3 half-lives
Molybdenum-99 has a half-life of 2.79 days.How much of a 16.80 mg sample of Mo-99 isleft after 8.37 days?
(this is the amount ofradioactive Mo-99 left;
i.e., 14.70 mg is now stable mat’l)
days 2.79days 8.37
start after one t1/2
after two t1/2
after three t1/2
16.80 mg8.40 mg
4.20 mg
2.10 mg
½ ½ ½ etc.
a. t1/2 = 5730 y, 3(5730 y) = 17190 y
t1/2 = 5730 y, 5(5730 y) = 28650 y
Example: Carbon-14 emits beta radiation and decays with a half-life of 5730y. Assume you start with a mass of 2.00g of carbon-14.
a. How long is 3 half-lives?b. How many grams will remain at the end of 3 half
lives?c. How many years will it take for only 0.0625g to
remain?
b. 2.00 g1
1.00 g2
0.50 g3
0.25 g
10.0625 g
20.125 g
30.25 g
40.50 g
51.00 gc. 2.00 g
t½ equation 2.00g
11.00g
20.50g
30.25g
2.00g(1/2)(1/2)(1/2)
N0(1/2)n = N
If 150.0 g of a radioactive substance undergoes 25 half lives, how many g will remain?
150.0 g (1/2)25 = 4.47 x 10 -6 g
or 2.00(1/2)3 = 0.25 g
N0 = initial amount
N = final amount
n = # of half lives
Energy Changes in Nuclear Reactions
Energy and mass aretwo sides of the same coin.
E = mc2
c = 3.00 x 108 m/sm = mass, in kgE = energy, in J
When a system loses/gains energy, it loses/gains mass.
In chemical reactions, this mass change is nearlyundetectable, so we speak of mass as being“conserved,” when it really isn’t. The amount of“mass-and-energy-together,” however, IS conserved.Mass changes in nuclear reactions are much largerthan in chemical reactions.
Nuclear Binding Energy
mass ofnucleus
mass of individualnucleons in nucleus <
rest masses: n0 = 1.00866 amu = 1.67493 x 10–24 g p+ = 1.00728 amu = 1.67262 x 10–24 g
mass defect = mass of
constituentnucleons
mass ofnucleus
–
This “missing” mass is convertedinto energy, which is used tohold the nucleus together.
(or “mass deficiency”)
“Tighter, lighter."
“Separate, heavier."
Use mass defect, E = mc2, and # of nucleons tocalculate binding energy per nucleon (BE/n).
-- large BE/n means great nuclear stability
-- BE/n is largest for Fe-56, meaning that nuclei __________ than Fe-56…
(1) LARGER…decay OR can undergo fission
(2) SMALLER…can undergo fusion
+ ENERGY
+ ENERGY
Both fission and fusion are exothermic.
Calculate the binding energy per nucleon of N-14,which has a nuclear mass of 13.999234 amu.
7 p+ (1.00728 amu) = 7.05096 amu7 n0 (1.00866 amu) = 7.06062 amu
14.11158 amu
m.d. = 14.11158 – 13.999234 = 0.11235 amu
0.11235 amu
= 1.8656 x 10–28 kg
= 1.20 x 10–12 J/nucleon
This is 7.50 x 106 eV, or 7.50 MeV.
amu 10 x 6.02g 1
23
g 1000kg 1
nBE
nmc2
14
)10 x (3 10 x 1.8656 2828
Most BE’s aremeasured in
electron-volts…1 eV = 1.60 x 10–19 J
Nuclear Fission
Fission requires… slow-moving neutrons.
distance too big;strong force weakens;+/+ repulsion takes over
fast n0slow n0released n0;free to splitmore nuclei
Important fissionable nuclei: U-233, U-235, Pu-239
chain reaction: one nuclear reaction leads to one or more others
critical mass: the mass of fissionable material requiredto maintain a chain reaction at a constant rate
safe safe criticalmass
(“Run, Forrest, run!”)
supercriticalmass
supercritical mass: the mass above which the chainreaction accelerates
(reactionmaintainedat constant
rate)
Little Boy, later dropped on Hiroshima(“Ah jes’ felt lahk runnING.”)
absorb n0,slow down the reaction
Nuclear Reactors
-- fuel is… ~3% U-235 (natural U is ~0.7% U-235)
-- control rods of B or Cd…
-- moderator: slows down n0 to cause fission
reactors in former USSR: graphite in the rest of the world: water
-- water is heated to steam, which spins electrical- generating turbines
low-molar-massmaterials
control room at anuclear power plant
WHY?
Schematic of a Nuclear Power Plant
EIW = Emergency Injection Water
PORV = Pressure Release Valve
Main benefits: (1) no air pollution; does NOTcontribute to global warming
(2) small volume of materialconsumed
(3) breeder reactors: reactors that generate new fissionable mat’l at a greater rate than the original fuel is consumed
-- non-fissionable U-238 is transmuted into fissionable Pu-239
Main problem: What to do with waste?
U-238 U-239 Pu-239 n0 +b
Np-239 b
Schematic ofBreeder Reactor
Nuclear Fusion
-- also called thermonuclear reactions
-- products are generally NOT radioactive
-- requires high temperatures
(> 40,000,000 K)
-- the tokamak uses magnetic fields to contain and heat the reaction
!!!!
WHY?
Radon
-- an a-emitter from the decay of radium in rocks and soil
Ra-226 a
Rn-222
-- very dense; seeps into basements and is readily inhaled
10%-- estimated to be responsible for ____ of U.S. lung cancer deaths
aPo-218
radonformed
radonemits a
radiation
this, too,is an
a-emitter
0 0