chapter 21 lecture- nuclear chemistry

Chapter 21Nuclear Chemistry

Nuclear Chemistry

The nucleus of an atomcontains

protons (+1charge)and

neutrons (no charge)

The nucleus is held togetherby the strong nuclear force

The strong nuclear force is the strongestforce known

Protons and neutrons are very closetogether

They exchange a teeny bit of mass backand forth.

When disrupted, the mass is convertedto energy according to E=mc2

The mass is tiny. The energy isimmense.

Protons and neutrons experiencethe strong nuclear force if close

enough

Because protons repel each otherthe nucleus needs a certain proton

to neutron ratio for stability

An unstable atom decays byemitting radiation

These unstable atoms are radioactive.Carbon-12 is stable. Carbon-14 is

radioactive.Carbon-14 is a radioisotope.There are many naturally occurring

radioisotopes and some that are human-made.

Radioisotopes decay intostable isotopes of a

DIFFERENT element

In nuclear reactions, the makeup of thenucleus changes.

Sometimes the number of protons willchange.

If the number of protons changes, theelement has changed.

The three most commonforms of radioactive decay are

alpha(α), beta(β) andgamma(γ)

Alpha particles are the leastpenetrating.

Gamma rays are the mostpenetrating.

25.1

To balance nuclear equationsyou must know the symbol for

the emitted particle

You must balance the atomic number (number onbottom) and the atomic mass (number on top)

Here 222 + 4 = 226 and 86 + 2 = 88 its balanced

The stuff that radiatesAn alpha particle is a helium nucleus it

has a +2 charge and a mass of 4amu.

A beta particle is an electronwhich is formed when a

neutron becomes a proton

A gamma ray is a high energyelectromagnetic wave.

A gamma ray has no mass orcharge so it is not in a nuclear

equation.

particle Alpha α Beta β Gamma γMass 4amu 0 0Charge +2 -1 0Effect Radioisotope

loses twoprotons and twoneutronsThe ELEMENTchanges

Radioisotopeconverts a neutronto a proton &ejects an electronThe ELEMENTchanges

Radioisotopeloses energyThe elementdoes notchange

What it is Heliumnucleus

electron High energyelectromagneticradiation

Stop it Paper/skin 1cm/metal foil Lead/concretedamage High ionization Medium

ionizationLowestionization

Damage from nuclearradiation is due to ionization of

living tissue.Nuclear radiation is called ionizing

radiation because it produces ions fromneutral molecules.

Alpha radiation has a low penetration, butit is the most damaging to living tissuebecause it deposits all its energy alonga short path

Nuclear Fission

Fission separates heavy elements into two lighterelements.

Uranium-235 Barium-141 + Krypton-92

+3neutrons

A huge amount of energy is produced.Used by humans as an energy sourceThe fission bomb was used in WWII

Nuclear fusion

Fusion combines two light nuclei into oneheavier element.

Produces even more energy than fission.Occurs in the sun.Requires extremely high temperatures

and pressures.

FUSION = to put togetherFISSION = to break apart

The Nucleus

• Remember that the nucleus is comprised ofthe 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.

Isotopes

• Not all atoms of the same element havethe same mass due to differentnumbers of neutrons in those atoms.

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

Radioactivity

• It is not uncommon for some nuclides ofan element to be unstable, orradioactive.

• We refer to these as radionuclides.• There are several ways radionuclides

can decay into a different nuclide.

Types ofRadioactive Decay

Alpha Decay:

Loss of an α-particle (a helium nucleus)

He42

U23892

→ U23490 He4

2+

Beta Decay:

Loss of a β-particle (a high energy electron)

β0−1 e0

−1or

I13153 Xe131

54→ + e0

−1

Positron Emission:

Loss of a positron (a particle that has thesame mass as but opposite charge thanan electron)

e01

C116

→ B115 + e0

1

Gamma Emission:

Loss of a γ-ray (high-energy radiationthat almost always accompanies the lossof a nuclear particle)

γ00

Electron Capture (K-Capture)

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

neutron.p1

1 + e0−1

→ n10

Neutron-Proton Ratios

• Any element with morethan one proton (i.e.,anything but hydrogen)will have repulsionsbetween the protons inthe nucleus.

• A strong nuclear forcehelps keep the nucleusfrom flying apart.

Neutron-Proton Ratios

• Neutrons play a key rolestabilizing the nucleus.

• Therefore, the ratio ofneutrons to protons is animportant factor.

Neutron-Proton Ratios

For smaller nuclei(Z ≤ 20) stablenuclei have aneutron-to-protonratio close to 1:1.

Neutron-Proton Ratios

As nuclei getlarger, it takes agreater number ofneutrons tostabilize thenucleus.

Stable Nuclei

The shaded region inthe figure showswhat nuclides wouldbe stable, the so-called belt of stability.

Stable Nuclei

• Nuclei above thisbelt have too manyneutrons.

• They tend to decayby emitting betaparticles.

Stable Nuclei

• Nuclei below the belthave too manyprotons.

• They tend tobecome more stableby positron emissionor electron capture.

Stable Nuclei

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

• These nuclei tend to decay by alphaemission.

Radioactive Series

• Large radioactivenuclei cannot stabilizeby undergoing onlyone nucleartransformation.

• They undergo a seriesof decays until theyform a stable nuclide(often a nuclide oflead).

Some Trends

Nuclei with 2, 8, 20,28, 50, or 82 protonsor 2, 8, 20, 28, 50,82, or 126 neutronstend to be morestable than nuclideswith a differentnumber of nucleons.

Some Trends

Nuclei with an evennumber of protonsand neutrons tend tobe more stable thannuclides that haveodd numbers ofthese nucleons.

Nuclear Transformations

Nucleartransformationscan be inducedby acceleratinga particle andcolliding it withthe nuclide.

Particle AcceleratorsThese particle accelerators are enormous,having circular tracks with radii that aremiles long.

Kinetics of Radioactive Decay

• Nuclear transmutation is a first-orderprocess.

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

= kt NtN0

ln

ln[A]t – ln[A]0 = -kt for 1st order kinetics

Kinetics of Radioactive Decay

• The half-life of such a process is:

= t1/2 0.693

k

• Comparing the amount of a radioactivenuclide present at a given point in timewith the amount normally present, onecan find the age of an object.

Not given, but canbe derived fromprevious equationwith [A]t equal tohalf of [A]0.

First order processes (including nuclear decay)always show a constant half-life

Measuring Radioactivity

• One can use a device like this Geiger counter tomeasure the amount of activity present in aradioactive sample.

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

Kinetics of Radioactive Decay

A wooden object from an archeological siteis subjected to radiocarbon dating. Theactivity of the sample that is due to 14C ismeasured to be 11.6 disintegrations persecond. The activity of a carbon sample ofequal mass from fresh wood is 15.2disintegrations per second. The half-life of14C is 5715 yr. What is the age of thearcheological sample?

Kinetics of Radioactive Decay

First we need to determine the rateconstant, k, for the process.

= t1/2 0.693

k

= 5715 yr 0.693k

= k 0.6935715 yr

= k 1.21 × 10−4 yr−1

Kinetics of Radioactive Decay

Now we can determine t:

= kt NtN0

ln

= (1.21 × 10−4 yr−1) t 11.615.2ln

= (1.21 × 10−4 yr−1) t ln 0.763= t 6310 yr

Energy in Nuclear Reactions

• There is a tremendous amount ofenergy stored in nuclei.

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

Energy in Nuclear Reactions

• In the types of chemical reactions wehave encountered previously, theamount of mass converted to energyhas been minimal.

• However, these energies are manythousands of times greater in nuclearreactions.

Energy in Nuclear Reactions

For example, the mass change for the decayof 1 mol of uranium-238 is −0.0046 g.The change in energy, ΔE, is then

ΔE = (Δm) c2

ΔE = (−4.6 × 10−6 kg)(3.00 × 108 m/s)2

ΔE = −4.1 × 1011 J

Nuclear Fission

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

out in nuclear reactors.

Nuclear Fission

• Bombardment of the radioactive nuclide witha neutron starts the process.

• Neutrons released in the transmutation strikeother nuclei, causing their decay and theproduction of more neutrons.

Nuclear Fission

This process continues in what we call anuclear chain reaction.

Nuclear Fission

If there are not enough radioactive nuclides in thepath of the ejected neutrons, the chain reactionwill die out.

Nuclear Fission

Therefore, there must be a certain minimumamount of fissionable material present for thechain reaction to be sustained: Critical Mass.

Fission reactor

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

Nuclear Reactors• The reaction is kept in

check by the use ofcontrol rods.

• These block the paths ofsome neutrons, keepingthe system from reachinga dangerous supercriticalmass.

Nuclear Fusion

• Fusion would be a superiormethod of generatingpower.The good news is that the

products of the reaction arenot radioactive.

The bad news is that inorder to achieve fusion, thematerial must be in theplasma state at severalmillion kelvins.

Nuclear Fusion

• Tokamak apparati like theone shown at the rightshow promise for carryingout these reactions.

• They use magnetic fieldsto heat the material.