NuclearChemistry

Nuclear ChemistryNuclear Chemistry

NuclearChemistry

Nuclear Reactions vs. Nuclear Reactions vs. Normal Chemical Normal Chemical

ReactionsReactions• Nuclear reactions involve the nucleusNuclear reactions involve the nucleus• The nucleus opens, and protons and The nucleus opens, and protons and

neutrons are rearrangedneutrons are rearranged• The opening of the nucleus releases a The opening of the nucleus releases a

tremendous amount of energy that tremendous amount of energy that holds the nucleus together – called holds the nucleus together – called binding energybinding energy

• ““Normal” Chemical Reactions involve Normal” Chemical Reactions involve electronselectrons, not protons and neutrons, not protons and neutrons

NuclearChemistry

The Nucleus

• Remember that the nucleus is comprised of the two nucleons, protons and neutrons.

• The nucleons are bound together by the strong force.

• The number of protons is the atomic number.• The number of protons and neutrons together is

effectively the mass of the atom.

NuclearChemistry

IsotopesAtoms of a given element with: same #protons

but different # neutrons

• Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms.

• There are three naturally occurring isotopes of uranium:Uranium-234Uranium-235Uranium-238

NuclearChemistry

H H H

http://education.jlab.org/glossary/isotope.html

NuclearChemistry

Radioactivity• Isotopes of certain unstable elements that

spontaneously emit particles and energy from the nucleus.

• So, some nuclides of an element are unstable, or radioactive.

• We refer to these as radionuclides.

• There are several ways radionuclides can decay into a different nuclide.

NuclearChemistry

Marie Curie a Pioneer of Radioactivity

• Lived in France• 1898 discovered the

elements polonium and radium.

• Winner of 1903 Nobel Prize for Physics with Henri Becquerel and her husband, Pierre Curie.

• Winner of the sole 1911 Nobel Prize for Chemistry.

NuclearChemistry

Types ofRadioactive Decay

NuclearChemistry

Alpha Decay:

Loss of an -particle (a helium nucleus) He42

U23892

U23490 He4

2+

helium nuclei two protons and two neutrons charge +2e  can travel a few inches through aircan be stopped by a sheet of paper, clothing.

NuclearChemistry

Alpha Decay

NuclearChemistry

Beta Decay:

Loss of a -particle (a high energy electron)

0−1 e0

−1or

I13153 Xe131

54 + e0

−1

Beta particles β: electrons ejected from the nucleus when neutrons decay ( n -> p+ + β - )

Beta particles have the same charge and mass as "normal" electrons.

•Can be stopped by aluminum foil or a block of wood.

NuclearChemistry

Gamma Emission:Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle) 00

Gamma radiation γ : electromagnetic energy that is released. Gamma rays are electromagnetic waves.They have no mass.Gamma radiation has no charge.

Most Penetrating, can be stopped by 1m thick concrete or a several cm thick sheet of lead.

NuclearChemistry

Positron Emission:

Loss of a positron (a particle that has the same mass as but opposite charge than an electron)

e01

C116

B115 + e0

1

NuclearChemistry

Electron Capture (K-Capture)

Addition of an electron to a proton in the nucleusAs a result, a proton is transformed into a

neutron.

p11 + e0

−1 n1

0

NuclearChemistry

Types of RadiationTypes of Radiation

e01

He42

• Alpha (Alpha (άά) – a positively ) – a positively charged helium isotopecharged helium isotope - - we we usually ignore the charge because it involves usually ignore the charge because it involves electrons, not protons and neutronselectrons, not protons and neutrons

•Beta (Beta (ββ) – an electron) – an electron

•Gamma (Gamma (γγ) – pure energy; ) – pure energy; called a ray rather than a called a ray rather than a particleparticle

00

NuclearChemistry

Other Nuclear ParticlesOther Nuclear Particles

e01

n10• NeutronNeutron

• Positron – a positive Positron – a positive electronelectron

•Proton – usually referred to Proton – usually referred to as hydrogen-1as hydrogen-1

•Any other elemental isotopeAny other elemental isotope

H11

NuclearChemistry

Examples of Radioactive Decay

Alpha Decay

Po Pb + He

Beta Decay p n + e

n p + e

C N + e

Gamma Decay

Ni Ni + (excited nucleus)

NuclearChemistry

Penetrating AbilityPenetrating Ability

NuclearChemistry

NuclearChemistry

NuclearChemistry

Balancing Nuclear ReactionsBalancing Nuclear Reactions•In the reactants (starting materials – on the left side of an equation) and products (final products – on the right side of an equation)

Atomic numbers must balanceand

Mass numbers must balance

•Use a particle or isotope to fill in the missing protons and neutrons

NuclearChemistry

Nuclear ReactionsNuclear Reactions

• Alpha emissionAlpha emission

Note that mass number (A) goes down by 4 and atomic number (Z) goes down by 2.

Nucleons (nuclear particles… protons and neutrons) are rearranged but conserved

NuclearChemistry

Nuclear ReactionsNuclear Reactions

• Beta emissionBeta emission

Note that mass number (A) is unchanged and atomic number (Z) goes up by 1.

NuclearChemistry

Other Types of Nuclear Other Types of Nuclear ReactionsReactions

Positron (Positron (00+1+1): a positive electron): a positive electron

Electron capture: Electron capture: the capture of an electron

207 207

NuclearChemistry

Learning Check

What radioactive isotope is produced in the following bombardment of boron?

10B + 4He ? + 1n

5 2 0

NuclearChemistry

Write Nuclear Equations!

Write the nuclear equation for the beta emitter Co-60.

NuclearChemistry

Part II

•Nuclear Stability

•Half-Life

NuclearChemistry

Nuclear Stability

For smaller nuclei (Z 20) stable nuclei have a neutron-to-proton ratio close to 1:1.

Depends on the neutron to proton ratio.

Neutrons play a key role stabilizing the nucleus.

Therefore, the ratio of neutrons to protons is an important factor.

NuclearChemistry

Neutron-Proton Ratios

As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.

NuclearChemistry

Band of Stability

Num

ber

of N

eutr

ons,

(N

)

Number of Protons (Z)

What happens to an unstable

nucleus?They will undergo decay

The type of decay depends on the reason for the instability

NuclearChemistry

Stable Nuclei• Nuclei above

this belt have too many neutrons.

• They tend to decay by emitting beta particles.

NuclearChemistry

Stable Nuclei

• Nuclei below the belt have too many protons.

• They tend to become more stable by positron emission or electron capture.

NuclearChemistry

Stable Nuclei

• There are no stable nuclei with an atomic number greater than 83.

• These nuclei tend to decay by alpha emission.

NuclearChemistry

Radioactive Series

• Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation.

• They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).

NuclearChemistry

Measuring Radioactivity

• One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample.

• The ionizing radiation creates ions, which conduct a current that is detected by the instrument.

NuclearChemistry

Half-LifeHalf-Life

•HALF-LIFEHALF-LIFE is the time that it takes is the time that it takes for 1/2 a sample to decompose.for 1/2 a sample to decompose.

• The rate of a nuclear transformation The rate of a nuclear transformation depends only on the “reactant” depends only on the “reactant” concentration.concentration.

NuclearChemistry

Half-LifeHalf-Life

Decay of 20.0 mg of Decay of 20.0 mg of 1515O. What remains after 3 half-lives? O. What remains after 3 half-lives? After 5 half-lives?After 5 half-lives?

NuclearChemistry

Kinetics of Radioactive Kinetics of Radioactive DecayDecay

For each duration (half-life), one half of the substance

decomposes.

For example: Ra-234 has a half-life of 3.6 days

If you start with 50 grams of Ra-234

After 3.6 days > 25 gramsAfter 3.6 days > 25 grams

After 7.2 days > 12.5 gramsAfter 7.2 days > 12.5 grams

After 10.8 days > 6.25 gramsAfter 10.8 days > 6.25 grams

NuclearChemistry

Half Life CalculationAE = Ao × 0.5t/t

1/2

AE = amount of substance left Ao = original amount of substance

t = elapsed time t1/2 = half-life of the substance

If you are given 157 grams of 14C, how much of 14C would be left after 2000 years? (The half-life of 14C is 5730 years.)

AE = 157 × 0.5(2000/5730)

Amount of 14C left after 2000 years would be 123 grams.

NuclearChemistry

Learning Check!

The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 39 hours?

NuclearChemistry

Kinetics of Radioactive Decay

• Nuclear transmutation is a first-order process.

• The kinetics of such a process, you will recall, obey this equation:

= kt Nt

N0

ln

NuclearChemistry

Kinetics of Radioactive Decay

• The half-life of such a process is:

= t1/2 0.693

k

• Comparing the amount of a radioactive nuclide present at a given point in time with the amount normally present, one can find the age of an object.

NuclearChemistry

Kinetics of Radioactive Decay

A wooden object from an archeological site is subjected to radiocarbon dating. The activity of the sample that is due to 14C is measured to be 11.6 disintegrations per second. The activity of a carbon sample of equal mass from fresh wood is 15.2 disintegrations per second. The half-life of 14C is 5715 yr. What is the age of the archeological sample?

NuclearChemistry

Kinetics of Radioactive Decay

First we need to determine the rate constant, k, for the process.

= t1/2 0.693

k

= 5715 yr 0.693

k

= k 0.693

5715 yr

= k 1.21 10−4 yr−1

NuclearChemistry

Kinetics of Radioactive Decay

Now we can determine t:

= kt Nt

N0

ln

= (1.21 10−4 yr−1) t 11.615.2

ln

= (1.21 10−4 yr−1) t ln 0.763

= t 6310 yr

NuclearChemistry

Energy in Nuclear Reactions

• There is a tremendous amount of energy stored in nuclei.

• Einstein’s famous equation, E = mc2, relates directly to the calculation of this energy.

NuclearChemistry

Energy in Nuclear Reactions

For example, the mass change for the decay of 1 mol of uranium-238 is −0.0046 g.

The change in energy, E, is then

E = (m) c2

E = (−4.6 10−6 kg)(3.00 108 m/s)2

E = −4.1 1011 J

NuclearChemistry

Nuclear FissionNuclear Fission

NuclearChemistry

Nuclear FissionNuclear FissionFission is the splitting of atomsFission is the splitting of atoms

These are usually very large, so that they are not as These are usually very large, so that they are not as

stablestable

Fission chain has three general steps:Fission chain has three general steps:

1.1. Initiation.Initiation. Reaction of a single atom starts the Reaction of a single atom starts the

chain (e.g., chain (e.g., 235235U + neutron)U + neutron)

2.2. PropagationPropagation. . 236236U fission releases neutrons U fission releases neutrons

that initiate other fissionsthat initiate other fissions

3. 3. ___________ ___________ . .

NuclearChemistry

Nuclear Fission

• How does one tap all that energy?• Nuclear fission is the type of reaction carried

out in nuclear reactors.

NuclearChemistry

Nuclear Fission

• Bombardment of the radioactive nuclide with a neutron starts the process.

• Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.

NuclearChemistry

Nuclear Fission

This process continues in what we call a nuclear chain reaction.

NuclearChemistry

Nuclear Fission & Nuclear Fission & POWERPOWER

• Currently about 103 Currently about 103

nuclear power plants nuclear power plants

in the U.S. and about in the U.S. and about

435 worldwide.435 worldwide.

• 17% of the world’s 17% of the world’s

energy comes from energy comes from

nuclear.nuclear.

NuclearChemistry

Nuclear Fission

If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out.

NuclearChemistry

Nuclear Fission

Therefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained: Critical Mass.

NuclearChemistry

Nuclear ReactorsIn nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator.

Diagram of a nuclear power plant

NuclearChemistry

Nuclear Reactors

• The reaction is kept in check by the use of control rods.

• These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass.

NuclearChemistry

Nuclear Fusion

Fusion small nuclei combine

2H + 3H 4He + 1n +

1 1 2 0

Occurs in the sun and other stars

Energy

NuclearChemistry

Nuclear Fusion

Fusion • Excessive heat can not be

contained• Attempts at “cold” fusion have

FAILED.• “Hot” fusion is difficult to contain

NuclearChemistry

Nuclear Fusion

• Fusion would be a superior method of generating power.The good news is that the

products of the reaction are not radioactive.

The bad news is that in order to achieve fusion, the material must be in the plasma state at several million kelvins.

NuclearChemistry

Nuclear Fusion

• Tokamak apparati like the one shown at the right show promise for carrying out these reactions.

• They use magnetic fields to heat the material.

NuclearChemistry

Radiocarbon DatingRadiocarbon DatingRadioactive C-14 is formed in the upper atmosphere Radioactive C-14 is formed in the upper atmosphere

by nuclear reactions initiated by neutrons in by nuclear reactions initiated by neutrons in cosmic radiationcosmic radiation

1414N + N + 11oon ---> n ---> 1414C + C + 11HH

The C-14 is oxidized to COThe C-14 is oxidized to CO22, which circulates , which circulates

through the biosphere.through the biosphere.

When a plant dies, the C-14 is not replenished.When a plant dies, the C-14 is not replenished.

But the C-14 continues to decay with tBut the C-14 continues to decay with t1/21/2 = 5730 = 5730

years.years.

Activity of a sample can be used to date the sample.Activity of a sample can be used to date the sample.

NuclearChemistry

Nuclear Medicine: Nuclear Medicine: ImagingImaging

Thyroid imaging using Tc-99mThyroid imaging using Tc-99m

NuclearChemistry

Food Food IrradiationIrradiation

•Food can be irradiated with Food can be irradiated with rays from rays from 6060Co or Co or 137137Cs.Cs.•Irradiated milk has a shelf life of 3 mo. Irradiated milk has a shelf life of 3 mo.

without refrigeration.without refrigeration.•USDA has approved irradiation of meats USDA has approved irradiation of meats

and eggs.and eggs.