Tuesday, May 6, 2008Tuesday, May 6, 2008

Questions on homeworkQuestions on homework Turn in homeworkTurn in homework Quick QuizQuick Quiz Continue on Chapter 19Continue on Chapter 19 Lab tomorrow (dress accordingly)Lab tomorrow (dress accordingly) Homework: Pg. 694 -- #12, 13, 14, Homework: Pg. 694 -- #12, 13, 14,

15, 23, 2615, 23, 26

Radioactive DecayRadioactive Decay

Radioactive nuclei spontaneously Radioactive nuclei spontaneously decompose forming a new nucleus decompose forming a new nucleus and one or more particlesand one or more particles

Cannot be turned on and offCannot be turned on and off Particles produced determine type of Particles produced determine type of

decaydecay

Alpha Particle DecayAlpha Particle Decay

Alpha (Alpha () particle decay) particle decay Common for heavy radioactive Common for heavy radioactive

nuclidesnuclides An alpha particle is really a helium An alpha particle is really a helium

nucleusnucleus This is the most massive type of This is the most massive type of

particle produced particle produced Can’t penetrate skin, but if ingested Can’t penetrate skin, but if ingested

does a lot of harm.does a lot of harm.

Beta Particle DecayBeta Particle Decay

Beta (Beta () particle decay) particle decay A A particle is another term for an particle is another term for an

electronelectron Beta particles have greater Beta particles have greater

penetrating ability than alpha penetrating ability than alpha particles, but are much less massiveparticles, but are much less massive

Gamma Ray ProductionGamma Ray Production Gamma (Gamma () decay) decay A A ray is a high-energy photon of light and is not ray is a high-energy photon of light and is not

a particlea particle This often accompanies decays of other types to This often accompanies decays of other types to

release excess energyrelease excess energy Gamma has the most penetrating ability, but Gamma has the most penetrating ability, but

because it has no mass or charge doesn’t interact because it has no mass or charge doesn’t interact with cells as readily to damage themwith cells as readily to damage them

It primarily takes the atom from an excited It primarily takes the atom from an excited energy state to a lower energy state with no energy state to a lower energy state with no change in the identity of the atomchange in the identity of the atom

Types of Radioactive DecayTypes of Radioactive Decay

Often a radioactive nucleus can’t achieve Often a radioactive nucleus can’t achieve a stable (nonradioactive) state through a a stable (nonradioactive) state through a single decay and so an entire decay series single decay and so an entire decay series is the result.is the result.

Figure 19.1: The decay series.Figure 19.1: The decay series.

Types of Radioactive DecayTypes of Radioactive Decay

Alpha (Alpha () particle decay) particle decay Common for heavy radioactive nuclidesCommon for heavy radioactive nuclides

Results in the loss of 4 in mass numberResults in the loss of 4 in mass number Loss of 2 in atomic numberLoss of 2 in atomic number Try to write the equation for the alpha Try to write the equation for the alpha

decay of thorium-230.decay of thorium-230.

RnHeRa 21886

42

22288

RaHeTh 22688

42

23090

Types of Radioactive Types of Radioactive DecayDecay

Beta (Beta () particle decay) particle decay A A particle is another term for an particle is another term for an

electron.electron. The net effect of The net effect of particle production is particle production is

to change a neutron to a proton and to change a neutron to a proton and electron. The proton increases Z, and electron. The proton increases Z, and the electron is released as a high energy the electron is released as a high energy particle. The mass # does not change.particle. The mass # does not change.

PaeTh 23491

01

23490

Types of Radioactive DecayTypes of Radioactive Decay

Try to write the equation for the beta Try to write the equation for the beta decay of Ac-227.decay of Ac-227.

TheAc 22790

01

22789

Types of Radioactive DecayTypes of Radioactive Decay

Gamma (Gamma () decay) decay A A ray is a high-energy photon of light ray is a high-energy photon of light

and is not a particle.and is not a particle. This often accompanies decays of other This often accompanies decays of other

types to release excess energy.types to release excess energy. It produces no net change in A or Z.It produces no net change in A or Z.

00

23490

42

23892 2ThHeU

Types of Radioactive Types of Radioactive DecayDecay

Positron decayPositron decay A positron is a particle with the same A positron is a particle with the same

mass as an electron, but the opposite mass as an electron, but the opposite charge.charge.

It has the effect of changing a proton to It has the effect of changing a proton to a neutron.a neutron.

The result is no change in mass #, and The result is no change in mass #, and a decrease in one of the atomic #.a decrease in one of the atomic #.

NeeNa 2210

01

2211

Types of Radioactive DecayTypes of Radioactive Decay

Try to write the equation for the Try to write the equation for the positron decay of K-38.positron decay of K-38.

AreK 3818

01

3819

Types of Radioactive DecayTypes of Radioactive Decay

Electron captureElectron capture Is the process in which one of the Is the process in which one of the

inner-orbital electrons is captured by inner-orbital electrons is captured by the nucleus.the nucleus.

Has a very similar effect to positron Has a very similar effect to positron emission.emission.

AueHg 20179

01

20180

Half-Lives Half-Lives and and

Radiocarbon DatingRadiocarbon Dating

Half LivesHalf Lives

Half Life: time required for half of the Half Life: time required for half of the original sample of nuclei to decayoriginal sample of nuclei to decay

If a certain radioactive sample If a certain radioactive sample contains 1000 nuclei at a given time contains 1000 nuclei at a given time and 7.5 days later has 500 nuclei, and 7.5 days later has 500 nuclei, then it has a half life of 7.5 days. then it has a half life of 7.5 days.

7.5 days later, it will have 250.7.5 days later, it will have 250.

Half LivesHalf Lives

A certain type of radioactive nuclide A certain type of radioactive nuclide always has the same half-life.always has the same half-life.

Half-lives can be as short as Half-lives can be as short as microseconds or as long as billions of microseconds or as long as billions of years.years.

Shorter half-lives indicate greater Shorter half-lives indicate greater activity.activity.

Half LivesHalf Lives

ExampleExample How many half lives does it take for How many half lives does it take for

1 mol to decay to 0.25 mol? 1 mol to decay to 0.25 mol? Two half-lives.Two half-lives.

Radiocarbon dating Radiocarbon dating or carbon-14 datingor carbon-14 dating Carbon-14 decays by Carbon-14 decays by -particle -particle

productionproduction

Carbon-14 is produced in the Carbon-14 is produced in the atmosphere when high-energy atmosphere when high-energy neutrons from space (cosmic rays) neutrons from space (cosmic rays) collide with nitrogen-14collide with nitrogen-14

NeC 147

01

146

HCnN 11

146

10

147

Carbon DatingCarbon Dating

Carbon-14 is continuously produced Carbon-14 is continuously produced in this process and it continuously in this process and it continuously decomposes. decomposes.

The two opposite processes have The two opposite processes have come into balance, so the amount of come into balance, so the amount of carbon-14 present in the atmosphere carbon-14 present in the atmosphere is approximately constant.is approximately constant.

Carbon DatingCarbon Dating Carbon-14 dating can be used to date Carbon-14 dating can be used to date

wood and cloth artifactswood and cloth artifacts A living plant consumes carbon dioxide in A living plant consumes carbon dioxide in

the photosynthesis process and has this the photosynthesis process and has this isotope in its molecules. isotope in its molecules.

While it is alive, the C-14 in the plant While it is alive, the C-14 in the plant molecules is equal to that in the molecules is equal to that in the atmosphere. atmosphere.

When the tree is cut, there is no more When the tree is cut, there is no more incoming source of C-14 to balance the C-incoming source of C-14 to balance the C-14 that is decaying. 14 that is decaying.

The C-14 content decreases.The C-14 content decreases.

Carbon DatingCarbon Dating

Using the known half-life of 5730 Using the known half-life of 5730 years, archaeologists compare the years, archaeologists compare the amount in an artifact to the amount amount in an artifact to the amount currently in trees and figure out the currently in trees and figure out the age. age.

A wooden bowl with half as much C-A wooden bowl with half as much C-14 as a current living tree would 14 as a current living tree would have been through one half life and have been through one half life and would therefore be 5730 years old.would therefore be 5730 years old.

Carbon Dating DiamondsCarbon Dating Diamonds

Diamonds were thought to be only created Diamonds were thought to be only created billions of years ago because no younger billions of years ago because no younger diamonds were being found. diamonds were being found.

Found a diamond in Zaire that is 628 million Found a diamond in Zaire that is 628 million years old- youngest diamond. years old- youngest diamond.

Diamonds take a long time to surface Diamonds take a long time to surface because they are only formed at depths of because they are only formed at depths of 200 km where the high pressures and 200 km where the high pressures and temperatures favor dense carbon formation. temperatures favor dense carbon formation.

Diamonds remain buried until they are Diamonds remain buried until they are brought to the surface by volcanic blasts.brought to the surface by volcanic blasts.