Chapter 19 Nuclear Chemistry

I. Properties of the NucleusA. Chemist’s View:

1) Seat of positive charge and mass in atoms and molecules

2) Not very important to chemical reactivity; valence electrons are key

B. Nuclear Characteristics

1) Very small size: about 1 x 10-13 cm (Whole atom = 1 x 10-8 cm)

2) Very high density: 1.6 x 1014 g/cm3

3) Very high energy processes (106 time greater than typical chemical reactions)

4) Components = “Nucleons”

a) Protons = +1 charge, 1 mass unit (Atomic Number = Z = # of protons)

b) Neutrons = 0 charge, 1 mass unit

c) Mass Number = A = sum of neutrons + protons

d) Isotopes = same atomic number but different mass numbers (#’s of neutrons)

e) Nuclide = a particular isotope

C C,C, X 146

136

126

AZ

II. Nuclear Stability and Radioactive DecayA. Thermodynamic Stability = potential energy of the nucleus compared to separate parts

B. Kinetic Stability = Probability that the nucleus will undergo Radioactive Decay

1) Example:

2) Both A and Z must be conserved (must be the same on both sides of equation)

3) Zone of Stability

a) All nuclides with Z > 84 unstable

b) (A-Z):Z ratio = 1 stable if light

c) (A-Z):Z ratio > 1 stable if heavy

d) Magic Numbers:

i. Z = even, (A-Z) = even stable

ii. Z = odd, (A-Z) = odd unstable

iii. Proton or Neutron numbers of

2, 8, 20, 28, 50, 82, 126 very stable

eNC 01-

147

146

C126

Hg20280

Calcium-40 is“Doubly Magic”

C. Types of Radioactive Decay

1) Decay involving the change in mass number of the nucleus

a) -particle production: loss of a helium nucleus; very common

b) Spontaneous Fission: splitting of a heavy nuclide into about equal parts; rare

2) Decay when mass number stays the same

a) -particle production: loss of an electron

i. Fairly common for nuclides where Neutrons:Protons > 1.0

ii. Nucleus doesn’t contain electrons; loss of energy that becomes electron

iii. Net effect: changes a neutron to a proton (Z increases by +1)

He Th U 42

23490

23892

He Ra Th 42

22688

23090

neutrons nuclideslighter Cf25498

e Pa Th 01-

23491

23490

e Xe I 01-

13154

13153

b) -ray production: loss of a high energy photon

i. Can accompany other decay types

ii. Way for nucleus in an excited state to return to ground state

c) Positron production: loss of mass of an electron, but positive charge

i. Occurs for nuclides with Neutron:Proton ratio < 1.0

ii. Net effect is change of a proton to a neutron (Z changes by -1)

iii. Positron is the Antiparticle of an Electron; collision with an electron leads to annihilation

d) Electron capture: an inner orbital electron is captured by the nucleus

i. Always produces -rays as well

ii. The ideal reaction for an alchemist, but too slow to be useful

00

42

23490

23892 2 He Th U

e Ne Na 01

2210

2211

00

01-

01 2 e e

00

20179

01-

20180 Au e Hg Examples

214

III. The Kinetics of Radioactive DecayA. Rate of Decay = - change in number of nuclides per unit time

1) Radioactive nuclides decay at a rate proportional to the size of the sample

2) This is the same as a first order rate law

3) Integrated first order rate law and half life equation work too!

4) Example: Technicium-99 is used for medical imaging. k = 0.116/h. t1/2 =?

5) Example: t1/2 of Molybdenum-99 is 67.0 h. How much of a 1.000 mg sample is left after 335 h?

kN N t

N- Rate

ktN

Nln

0

k

693.0t1/2

h 97.50.116/h

693.0

k

693.0t1/2

0.032mg)000.1()e(NNeN

Nkt

N

Nln )335)(/0103.0(kt

0kt

00

hhemg

/h0103.067.0h

0.693

t

0.693k

k

693.0t

1/21/2

B. Carbon Dating

1) Archeological technique to determine the age of artifacts

2) Willard Libby received the Nobel Prize in Chemistry for this work

3) Based on the radioactive decay of carbon-14

4) Carbon-14 is continuously produced in the atmosphere by neutrons from space

a) These processes have reached equilibrium: no net change in [carbon-14]

b) Plants take up the carbon as CO2 while alive, but stop when they die

c) Ratio of 14C to 12C begins to get smaller as soon as the plant dies

d) t1/2 = 5730 years for the decay of 14C

5) Example: 14C decay is 3.1/min. Fresh wood is 13.6/min. t1/2 = 5730 y.

e N C 01-

147

146

H C n N 11

146

10

147

/y10 x 1.215730y

0.693

t

0.693k

k

693.0t 4

1/21/2

y 12,000/y10 x 1.21

13.63.1

ln

k

NN

ln

tktN

Nln

40

0

IV. Applications of Nuclear ReactionsA. Nuclear Transformations

1) Particle accelerators: device to propel particles at high speed

a) Linear accelerator uses changing electric fields

b) Cyclotron uses oscillating voltage to accelerate; magnets cause circular path

2) Bombarding Nuclides with other nuclides or particles can lead to new Nuclides

3) Most of the “trans-Uranium” elements were synthesized this way (Z = 93-112)

a) Neutron Bombardment

b) Positive-Ion Bombardment

B. Medical Uses

1) Radiotracers = radioactive nuclides introduced to an organism to follow pathway

a) Iodine-131 is used to diagnose thyroid gland problems

b) Thallium-201 and Technetium-99 diagnose heart damage

2) PET scan = Positron Emission Tomography

e Np n U 01-

23993

10

23892

n Cm He U 10

24296

42

23994

Targeted Imaging: PET

radiopharmDRUG

C. Energy Production

1) Fission = splitting a heavy nuclide into 2 lighter, more stable ones (H = -)

a) Uranium fission provides electrical power

b) 3.5 x 10-11 J/nuclide = 2.1 x 1013 J/mol of energy is given off by loss of mass

c) E = mc2 is used to calculate the amount of energy from the mass loss

d) Chain reaction: neutrons produced can cause more reactions

i) Subcritical: < 1 neutron/reaction causes another fission (rxn dies out)

ii) Critical: = 1 neutron/reaction causes another fission (rxn sustained)

iii) Supercritical: > 1 neutron/reaction causes another fission (explosion)

e) Nuclear Reactor: Fission heats water, runs turbine, make electricity

i) Reactor core: enriched uranium (3% U-235) sustains the reaction

ii) Control rods absorb neutrons to regulate the reaction

f) Breeder Reactor: produces its own fissionable Pu-239 from U-238

Pu-239 is toxic and flames in air, so U.S. doesn’t use, France does

n3 Kr Ba n U 10

9236

14156

10

23592

2) Fusion = combining 2 light nuclides to form a heavier, more stable one (H = -)

a) Stars produce their heat through this process

b) Would be great energy source on Earth

i. Lots of small nuclei to use as fuel

ii. But, only takes place at high temperatures (40,000,000 Kelvins)

iii. High temperature overcomes strong nuclear repulsion (+/+)

iv. E = mc2 (4.03298 amu in; 4.00260 amu out)

D. Effects of Radiation

1) Damage to organisms

a) Somatic damage = damage to the organisms itself (sickness or death)

b) Genetic damage = damage to genetic material (offspring are effected)

2) Factors controlling radiation effects

a) Energy of the radiation: higher energy = more damage (1 Rad = 0.01 J/kg)

b) Penetrating ability: -ray > -particle (1cm) > -particle (stopped by skin)

c) Ionizing ability: removing electrons; -particle >> -ray

d) Chemical properties: Kr-85 inert, excrete quickly; Sr-90 replaces Ca, stays

He H H

e H H H32

21

11

01

21

11

11

e He H He

H2 He He He01

42

11

32

11

42

32

32

3) REM = Roentgen Equivalent for Man = normalizes radiation effects for different types of radiation exposure

a. Short term effects of radiation exposure

b. There are natural and man-made sources of radiation exposure

4) Models for radiation exposure damage

a. Linear model: any exposure is bad, minimize all exposures

b. Threshold model: no damage unless a certain amount of exposure occurs

c. Better safe than sorry: we don’t know which model is correct, follow linear

REM