NuclearChemistry

Chapter 21Nuclear Chemistry

Chemistry, The Central Science, 10th editionTheodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten

John D. BookstaverSt. Charles Community College

St. Peters, MO 2006, Prentice Hall, Inc.

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February 3

• Nuclear chemistry

• HW

• 1,2,3,7,11,13,17,19,27,29 for tomorrow

• 31 to35 odd, 41,57,59,61

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The Nucleus

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

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

is effectively the mass of the atom.

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Isotopes

• 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

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Radioactivity

• It is not uncommon for some nuclides of an element to be unstable, or radioactive.

• We refer to these as radionuclides.

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

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Types ofRadioactive Decay

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SeparationAlphaBetaGamma.MOV

Separation of Radiation

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Nuclear Reactions• The chemical properties of the nucleus are

independent of the state of chemical combination of the atom.

• In writing nuclear equations we are not concerned with the chemical form of the atom in which the nucleus resides.

• It makes no difference if the atom is as an element or a compound.

• Mass and charges MUST BE BALANCED!!!

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Alpha Decay:

Loss of an -particle (a helium nucleus)

He42

U23892

Th23490 He4

2+

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Alpha Decay

• Mass changes by 4

• The remaining fragment has 2 less protons

• Alpha radiation is the less penetrating of all the nuclear radiation (it is the most massive one!)

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Beta Decay:

Loss of a -particle (a high energy electron)

0−1 e0

−1or

I13153 Xe131

54 + e0

−1

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Beta Decay

• Involves the conversion of a neutron in the nucleus into a proton and an electron.

• Beta radiation has high energies, can travel up to 300 cm in air.

• Can penetrate the skin

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Beta decay

• Write the reaction of decay for C-14

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Gamma Emission:

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

00

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Positron Emission:

Loss of a positron ( particle with same mass, but opposite charge than an electron)

e +01

C116

B115 + e 0

1

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Positron emission

• Involves the conversion of a proton to a neutron emitting a positron.

• The atomic number decreases by one, mass number remains the same.

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Electron Capture (K-Capture)Capture by the nucleus of an electron from the

electron cloud surrounding the nucleus.

As a result, a proton is transformed into a neutron.

p11 + e0

−1 n1

0

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Electron capture

• Rb-81

• Note that the electron goes in the side of the reactants. Electron gets consumed.

Rb8137

Kr81

36

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Patterns of nuclear Stability

• Any element with more than one proton ( all but hydrogen) will have repulsions between the protons in the nucleus.

• A strong nuclear force helps keep the nucleus from flying apart.

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Neutron-Proton Ratios

• Neutrons play a key role stabilizing the nucleus.

• The ratio of neutrons to protons is key to determine the stability of a nucleus .

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Neutron-Proton Ratios

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

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Neutron-Proton Ratios

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

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Stable Nuclei

The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.

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Stable Nuclei

• Nuclei above this belt have too many neutrons.

• They tend to decay by emitting beta particles. ( neutron becomes proton )

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Above the belt of stabilityBeta particle emission

• Too many neutrons. The nucleus emits Beta particles, decreasing the neutrons and increasing the number of protons.

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Stable Nuclei

• Nuclei below the belt have too many protons.

• They tend to become more stable by positron emission or electron capture (both lower the number of protons)

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Stable Nuclei

• Elements with low atomic number are stable if # proton = # neutrons

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

• These nuclei tend to decay by alpha emission.

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Below the stability beltIncrease the number of neutrons

(by decreasing # protons)

• Positron emission more common in lighter nuclei.

• Electron capture common for heavier nuclei.

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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).

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Predicting modes of nuclear decay

C-14

Xe-118

Pu-239

In-120

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• beta decay

• Positron emission or electron capture

• Alpha decay (too heavy, loses mass)

• Beta decay (ratio too low, gains protons)

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MAGIC NUMBERS 2, 8, 20, 28, 50, or 82

Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons tend to be more stable than nuclides with a different number of nucleons.

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Some Trends

Nuclei with an even number of protons and neutrons tend to be more stable than nuclides that have odd numbers of these nucleons.

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Shell model of the nucleus

• Nucleons are described a residing in shells like the shells for electrons.

• The numbers 2,8,18,36,54,86 correspond to closed shells in nuclei.

• Evidence suggests that pair of protons and pairs of neutrons have special stability

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Transmutations

• To change one element into another.

• Only possible in nuclear reactions never in a chemical reaction.

• In order to modify the nucleus huge amount of energy are involved.

• These reactions are carried in particle accelerators or in nuclear reactors

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Nuclear transmutations

• Alpha particles have to move very fast to overcame electrostatic repulsions between them and the nucleus.

• Particle accelerators or smashers are used. They use magnetic fields to accelerate the particles.

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Particle Accelerators(only for charged particles!)

These particle accelerators are enormous, having circular tracks with radii that are miles long.

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Cyclotron

Nuclear transformations can be induced by accelerating a particle and colliding it with the nuclide.

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Neutrons

• Can not be accelerated. They do not need it either (no charge!).

• Neutrons are products of natural decay, natural radioactive materials or are expelled of an artificial transmutation.

• Some neutron capture reactions are carried out in nuclear reactors where nuclei can be bombarded with neutrons.

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Representing artificial nuclear transmutations

• 14N + 4He 7O + 1H

Target nucleus ( bombarding particle, ejected particle ) product nucleus

• 14N (p) 17O • Write the balanced nuclear equations

summarized as followed:• 16 O ( p,

• 27Al (n, a

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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.

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Mass defect• The mass of the nucleus is always

smaller than the masses of the individual particles added up.

• The difference is the mass defect.

• That small amount translate to huge amounts of energy E = (m) c2

• That energy is the Binding energy of the nucleus, and is the energy needed to separate the nucleus.

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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 This amount is 50,000 times

greater than the combustion of 1 mol of CH4

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Types of nuclear reactionsfission and fusion

• The larger the binding energies, the more stable the nucleus is toward decomposition.

• Heavy nuclei gain stability (and give off energy) if they are fragmented into smaller nuclei. (FISSION)

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• Even greater amounts of energy are released if very light nuclei are combined or fused together. (FUSION)

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Nuclear Fission

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

out in nuclear reactors.

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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.

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Nuclear Fission

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

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Nuclear Fission

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

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Nuclear Fission

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

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Controlled vs Uncontrolled nuclear reaction

• Controlled reactions: inside a nuclear power plant

• Uncontrolled reaction: nuclear bomb

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Nuclear Reactors

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

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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.

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FUSION

• Combining small nucleii to form a larger one.

• Require millions of K of temperature

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Fusion

• 1H + 1H 2H + 1e + energy

• 1H + 2H 3He + energy

• 3He + 3He 4He + 21H + energy

• Reaction that occurs in the sun

• Temperature 107 K

• Heavier elements are synthesized in hotter stars 108 K using Carbon as fuel

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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.

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Nuclear Fusion(thermonuclear reactions)

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

• They use magnetic fields to heat the material.

• 3 million K degrees were reached inside but is not enough to begin fusion which requires 40 million K

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Rates of radioactive decayrate = k N

N is the number of radioactive nuclei

• Activity: rate at which a sample decays. Expressed in disintegrations per unit time.

• Becquerel (Bq) SI unit : one nuclear disintegration per second.

• Curie (Ci) 3.7x1010 disintegrations per second, the rate of decay of 1g of Ra

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RADIOACTIVE DECAY

• As a radioactive sample decays, the amount of radiation emanating for the sample decays as well.

• After one half life, half of the emanations!

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Half-Life

• Half-life is defined as the time required for one-half of a reactant to react.

• Because [A] at t1/2 is one-half of the original [A],

[A]t = 0.5 [A]0.

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RADIOACTIVE DECAYIs a first order process. Its rate is proportional to

the number of radioactive nuclei N in the sample rate= k N

N0

Nt

ln = kt

Time elapsed =t k is the decay constantN0 is the original amountNt is the amount of sample at time t

0.693 = kt1/2

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Half life

• The half life of a reaction is useful to describe how fast it occurs.

• For a first order reaction (like nuclear decay!) it does not depend on the initial concentration of the reactants.

• HALF LIFE IS CONSTANT FOR A FIRST ORDER REACTION

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Half LifeDecay of 10.0 g sample of Sr-90

t1/2= 28.8 y

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Problem 1

• The half life of 210Pb= 25 y

1) How much left of a sample of 50 mg will remain after 100 y?

• 2) Find number of half lives

• 3) Find fraction left

 

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• 1- 6.25 g

• 2- 4 half lives

• 3- 1/16

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Problem 2

• How many years will take for 50mg of 210Pb to decay to 5 mg?

• Half life of 210Pb= 25 y

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• 83 years

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Problem 3

• 90 % of a radioisotope disintegrates in 36 hs. What is the half life?

•  

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Problem 4

 

• .953 g of Sr-90 remains after 2 y from a 1.000g sample.

• a) find the half life

• b) how much will remain after 5 y?

•  

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• Half life = 28.8 years

• Amount left (No) = 0.89 g

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Radioactive Dating

• A rock contains .257 mg of Pb-206 for every mg of U-238.

• T1/2 = 4.5 x 10 9 y

• How old is the rock?

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• No = we will assume that all the Pb-206 that is now present will come from the original U, plus the U that is still present

• (check the answer in textbook!)

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Calculating half life:

• If 87.5 % of a sample of I-131 decays in 24 days, what is the half life of the

I-131?