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