NuclearNuclear ChemistryChemistry

Chapter 10Chapter 10

10.1 Radioactivity10.1 Radioactivity

Becquerel…French guy….discovered Becquerel…French guy….discovered radiation.radiation.Noticed uranium salts would glow after being Noticed uranium salts would glow after being

exposed to light.exposed to light.Thought glow was X-rays.Thought glow was X-rays.Tested….kept salt on top of photographic Tested….kept salt on top of photographic

plates wrapped with paper. Left in sun.plates wrapped with paper. Left in sun.Plates became fogged.Plates became fogged.

Nuclear DecayNuclear DecayRadioactivity => Process in which an unstable Radioactivity => Process in which an unstable

atomic nucleus emits charged particles and atomic nucleus emits charged particles and energy. energy.

Unstable nucleus = radioactive isotope = radioisotopeUnstable nucleus = radioactive isotope = radioisotope Radioisotopes Radioisotopes spontaneouslyspontaneously change into other change into other

isotopes over time.isotopes over time. In other words, atoms of one element become atoms of a In other words, atoms of one element become atoms of a

different element.different element.

Types of Nuclear Decay

Alpha decay Beta decay Gamma decay

Alpha decay

Positively charge-2 protons & 2 neutrons

42He superscript is mass #

Subscript is atomic #

or ‘alpha’

Least penetrating of the 3 types. Stopped by clothes or sheet of paper.

23892

U ------> 23490Th + 4

2He

Beta decay

Is an electron released from inside an unstable nucleus.

o-1e or so small…virtually massless

A neutron changes into a proton & gives away an electron.

23490Th ------> 234

91 Pa + o

-1e

Penetrates more than particles.Goes through paper, but stopped by

thin metal foil.

Gamma decay

Not a particle, but energy emitted from an unstable nucleus.

No mass, no charge. Energy waves that travel at speed of light.

Often happens at the same time alpha & beta decay. 234

90Th ------> 23491 Pa + o

-1e +

Most penetrating radiation.

or gamma ray

Takes several cm of lead or several meters of concrete to stop

SOURCESOF RADIATION?

Background radiation-air

-rocks-water-plants

-animalsLow enough that doesn’t damage.

Cosmic radiation-from Sun &

Outer space

Radiation that exceeds background radiation Radiation that exceeds background radiation damages cells & tissues.damages cells & tissues.

Nuclear radiation can ionize atomsNuclear radiation can ionize atoms.. Within cells, bonds hold proteins & DNA molecules Within cells, bonds hold proteins & DNA molecules

together. Broken by radiation.together. Broken by radiation. Cells cannot function normally.Cells cannot function normally. Radon-222Radon-222 gas coming from decay of uranium under gas coming from decay of uranium under

buildings can buildings can seep into basementsseep into basements. Stays in air for . Stays in air for 3.82 days.3.82 days.

When breathed in, emits alpha particles. When breathed in, emits alpha particles. 22ndnd leading cause of lung cancer leading cause of lung cancer.. Test kits at health department.Test kits at health department.

Nuclear Radiation cannot be seen, heard or felt.Nuclear Radiation cannot be seen, heard or felt. Geiger counters and film badges can detect radiation.Geiger counters and film badges can detect radiation.

10.2 Rates of Nuclear Decay10.2 Rates of Nuclear Decay

Nuclear decay rate has to do with how Nuclear decay rate has to do with how fastfast nuclear changes takes place. nuclear changes takes place.All radioisotopes decay at a specific rate.All radioisotopes decay at a specific rate.Half-lifeHalf-life=> Time required for ½ of the => Time required for ½ of the

radioisotope to decay.radioisotope to decay.After 1 half-life, ½ the original amount remains.After 1 half-life, ½ the original amount remains.Two ½ lives……one quarter or 25% remains.Two ½ lives……one quarter or 25% remains.

½ lives vary from fractions of a ½ lives vary from fractions of a secondsecond to to billionsbillions of years! of years!

Iodine-1314.0 grams

½ life of 8.07 daysOnly 2.0 grams left.

16.14 days….Only 1.0 grams left.

24.21 days……._____g leftover

After_________ day, _____ grams is leftover

1st ½ life

2nd ½ life

3rd ½ life

4th ½ life

Half-lives & Radiation typesHalf-lives & Radiation types

Radon-222Radon-222 3.82 days3.82 days Iodine-131Iodine-131 8.07 days8.07 days Thorium-234Thorium-234 24.1 days24.1 days Radium-226Radium-226 1,620 years1,620 years Carbon-14Carbon-14 5,730 years5,730 years Thorium-230Thorium-230 75,200 years75,200 years Uranium-235Uranium-235 7.04x107.04x1088 years years Potassium-40Potassium-40 1.28x101.28x1099 years years Uranium-238Uranium-238 4.47x104.47x109 9 yearsyears

UnlikeUnlike chemical reaction rates, which vary chemical reaction rates, which vary with conditions, nuclear decay rates are with conditions, nuclear decay rates are always the same no matter what.always the same no matter what.

Radioisotopes decay in different ways….Radioisotopes decay in different ways….alpha, beta, gamma or combinations of them.alpha, beta, gamma or combinations of them.

Iridium-182 goes through beta decay into Iridium-182 goes through beta decay into Osmium-182.Osmium-182. Ir-182 -----> Os-182 + Ir-182 -----> Os-182 + 00

-1-1ee½ life is 15 min ½ life is 15 min After 45 minutesAfter 45 minutes

what fraction is leftover? # half lives?what fraction is leftover? # half lives?

Radioactive datingRadioactive datingFinding-out how old things are, carbon-14 is Finding-out how old things are, carbon-14 is

typically what is checked.typically what is checked.C-14 has a ½ life of 5,730 years.C-14 has a ½ life of 5,730 years. It reacts with Oxygen to make carbon dioxide.It reacts with Oxygen to make carbon dioxide.

Plants take-in COPlants take-in CO22 during photosynthesis. during photosynthesis.Animals eat plants.Animals eat plants.After plants or animals die, C-14 no longer is After plants or animals die, C-14 no longer is

ingested.ingested.The age of any object that once lived can then be The age of any object that once lived can then be

found by comparing its C-14 levels with what is found by comparing its C-14 levels with what is already in the air or objects with a known age.already in the air or objects with a known age.

Carbon dating only works for objects that Carbon dating only works for objects that are less than 50,000 years old.are less than 50,000 years old.

More than 50,000 years, must go with More than 50,000 years, must go with other radioisotopes like K-40, U-235, other radioisotopes like K-40, U-235,

U-238.U-238.

10.4 10.4 Fission & FusionFission & Fusion

Nuclear reactions involve converting Nuclear reactions involve converting to different elements…but also into to different elements…but also into pure energy.pure energy.

Nucleus should fly apart. Too much Nucleus should fly apart. Too much repulsion between protons.repulsion between protons.• Like bringing to magnets together.Like bringing to magnets together.

This force is called the This force is called the Electric forceElectric force..

Scientists believed Scientists believed that an attractive that an attractive force must exist to force must exist to oppose the electric oppose the electric force.force.

This force is called This force is called the the StrongStrong forceforce..

Repulsion = Repulsion = ElectricElectric forceforce

Attraction = Attraction = StrongStrong forceforce

The The Strong forceStrong force binds protons & neutrons binds protons & neutrons together in the nucleus.together in the nucleus.

Only exist Only exist insideinside the nucleus. the nucleus.• Acts then over a very, very small distance.Acts then over a very, very small distance.

Size is a factor.Size is a factor.• Small nuclei have Small nuclei have lessless protons, so less protons, so less ElectricElectric force. force.• LargeLarge nuclei have more protons, so nuclei have more protons, so moremore ElectricElectric force. force.

Electric force increases as the nucleus gets Electric force increases as the nucleus gets bigger, the strong force increases…but only to a bigger, the strong force increases…but only to a point.point.• Once it gets too big, then the distance between particles Once it gets too big, then the distance between particles

is too great and the strong force is no different than is too great and the strong force is no different than small nuclei.small nuclei.

In summary, the strong force does NOT increase In summary, the strong force does NOT increase with radius size, but the with radius size, but the ElectricElectric force does. force does.

See Figure 16 on p.309See Figure 16 on p.309

When a nucleus is When a nucleus is too big, then the too big, then the strong force cannot strong force cannot overcome the overcome the electric force.electric force.

The nucleus is then The nucleus is then ‘‘unstableunstable’ or ’ or radioactive!radioactive!

All nuclei with 83 All nuclei with 83 or more protons or more protons are unstable.are unstable.

FissionFission

1939, two German 1939, two German scientists were the 1scientists were the 1stst to split a nucleus into to split a nucleus into two smaller particles.two smaller particles.• U-235 U-235

Became referred to as Became referred to as fissionfission..• ionion means ‘the act of’. means ‘the act of’.

• fissfiss means to ‘split’. means to ‘split’.

U-235 is hit by a neutron and splits U-235 is hit by a neutron and splits into Kr-91, Ba-56 & 3 more neutrons.into Kr-91, Ba-56 & 3 more neutrons.• Also releases a LARGE amount of energy.Also releases a LARGE amount of energy.

The fission of 1 kg of U-235 produces The fission of 1 kg of U-235 produces more energy than 17,000 kg of coal!more energy than 17,000 kg of coal!

Scientists found that 0.1% of the U-Scientists found that 0.1% of the U-235 mass is lost…..where’d it go??235 mass is lost…..where’d it go??

Einstein figured it out 30 years Einstein figured it out 30 years before.before.

EE = = mmcc22 is a very famous equation. is a very famous equation.• Really means Energy and mass are the Really means Energy and mass are the

same thing.same thing. Demonstrated in 1945 with 1Demonstrated in 1945 with 1stst atomic atomic

bomb.bomb.• 5 kg of Pu-239. 5 kg of Pu-239. • Explosion was same as Explosion was same as 18,60018,600 tonstons of of

TNT.TNT.• Huge!!!!!!!!!!!!!!!!!!!!!Huge!!!!!!!!!!!!!!!!!!!!!

Chain reactionChain reaction• Neutrons released during the splitting of an Neutrons released during the splitting of an

initial nucleus trigger a series of nuclear initial nucleus trigger a series of nuclear fissions.fissions.

U-235 is split into two separate nuclei and each time U-235 is split into two separate nuclei and each time this happens, two to three neutrons are released. this happens, two to three neutrons are released. Those neutrons then hit other U-235 nuclei and the Those neutrons then hit other U-235 nuclei and the process continues.process continues.

• Rate of a chain reactions vary.Rate of a chain reactions vary. Fast & intense release of energy in nuclear weapons.Fast & intense release of energy in nuclear weapons. Slow & controlled reactions Slow & controlled reactions

• Nuclear reactors slow down chain reactions by Nuclear reactors slow down chain reactions by absorbing extra neutrons.absorbing extra neutrons.

Critical massCritical mass=> The smallest mass needed in order => The smallest mass needed in order to have a sustained chain reaction.to have a sustained chain reaction.

FusionFusion

FusionFusion =>Nuclear reaction in which =>Nuclear reaction in which the nuclei of the nuclei of twotwo atoms combine to atoms combine to

form a form a largerlarger nucleus. nucleus. Like in fission, a very small amount Like in fission, a very small amount

of mass will convert to energy.of mass will convert to energy. Occurs inside the sun.Occurs inside the sun.

• 600 million tons/second…4 million of it 600 million tons/second…4 million of it into pure energy.into pure energy.

Needs really high temperatures to Needs really high temperatures to occur.occur.• In the sun, temperatures reach In the sun, temperatures reach

10,000,000 10,000,000 ooC.C.• Temperatures this high cannot be Temperatures this high cannot be

contained inside of anything here on contained inside of anything here on earth.earth.

Fusion may someday provide an Fusion may someday provide an efficient & clean source of electricity.efficient & clean source of electricity.