Chapter 20: Radioactivity & Nuclear Chemistry

Ch 20.1 of 64

20.1 Introduction • We’ve investigated various ways in which chemistry involves energy changes – Thermodynamics – Electrochemistry

• Nuclear chemistry and radioactivity concern energy changes due to reactions occurring within the nucleus • What is radioactivity? • How do changes in the nucleus release energy? • What applications does nuclear chemistry have? Ch 20.2 of 64

20.2 The Discovery of Radioactivity • Antoine-Henri Bequerel discovered photographic film became dark when KUSO4 crystals were close by • He realized that ‘uranyl rays’ were being released from the crystals continuously • Marie Curie discovered that Po and Ra emitted ‘uranyl rays’ too and changed the name to radiaoctivity

Radium-coated surfaces release energy; they glow visibly and emit heat

Ch 20.3 of 64

20.3 Types of Radioactivity • Ernest Rutherford and others characterized the emission released by radioactive elements as they attempt to reduce their potential energy • Natural radioactivity is classified as: • Alpha (α) emission/decay • Beta (β) emission/decay • Gamma (γ) ray emission/decay • Positron emission/decay • Sometimes nuclei absorb an electron (electron capture) Ch 20.4 of 64

Atomic Notation • Recall the notation used to describe isotopes Mass number Atomic number

A Z

Xx

Element Symbol

• Mass number (A) is the sum of neutrons and protons in the nucleus • Atomic number (Z) is the number of protons Number of neutrons

N=A-Z

• Number of electrons is the same as the number of protons in neutral atoms Ch 20.5 of 64

Nuclides and Isotopes • In the same notation 1 1

p

1 0

n

0 −1

e

Not standard but useful notation used later

proton neutron electron • Isotopes have same atomic number (same number of protons) but a different number of neutrons (different mass number) – 20Ne has 10 protons, 10 electrons, 10 neutrons – 21Ne has 10 protons, 10 electrons, 11 neutrons – Sometimes written as Ne-20 and Ne-21

• Isotopes are sometimes called nuclides

Ch 20.6 of 64

Alpha (α) Emission • An alpha particle contains 2 protons and 2 neutrons, identical to a He nucleus (He2+) 4 2

Alpha particle :

He

• A nuclear equation describes a nuclear reaction Parent nuclide

238 92

U → He + 4 2

234 90

Th

Daughter nuclide

• Note the equation must be balanced – Number of protons and neutrons Ch 20.7 of 64

Alpha (α) Emission • Write the nuclear equation for the alpha decay of Po-216 – We must deduce the identity of the daughter nuclide 216 84

Po → 42 He + ?? ??

• Adaughter = 216 - 4 = 212 • Zdaughter = 84 - 2 = 82 (Pb) 216 84

Po → 42 He +

212 82

Pb

Note that α emission changes the identity of the element!

Ch 20.8 of 64

Alpha (α) Emission • The α particle is the most massive radioactive particle • It interacts strongly with other atoms and molecules • It has a high ionizing potential because it is charged (He2+), causing ionization in target atoms • It has a low penetrating potential because it is massive, easily stopped by few mm of air or a sheet of paper • Causes little damage external to the body but may cause significant damage to cells if ingested Ch 20.9 of 64

Beta (β) Emission • A beta particle is equivalent to an electron emitted from the nucleus of the atom

Beta particle : • Beta decay can be written

n → 11p + −10e

1 0

0 −1

e

The -1 represents the charge of the electron and allows equation to balance

• The β particle leaves the nucleus and the proton remains Ch 20.10 of 64

Beta (β) Emission • The remaining nuclide now has an additional proton 228 228 0 Ra → Ac + 88 89 −1e • Beta decay also changes the identity of the nuclide • Note that the electron comes from the nucleus, not a core or valence electron! 228 88

Ra →

228 88

Ra + + e − Ch 20.11 of 64

Beta (β) Emission • The β particle is much less massive than an α particle • It has moderate ionizing potential because it is singly charged (e-), causing less ionization in target atoms • It has moderate penetrating potential because it is small, stopped by few mm or metal or few cm of wood • Causes moderate damage external to the body but less damage to cells if it is ingested because it causes little ionization Ch 20.12 of 64

Gamma (γ) Ray Emission • A gamma ray is short wavelength electromagnetic radiation emitted from the nucleus of the atom

Gamma ray :

0 0

γ

• Gamma rays have no mass or charge and so do not alter the identity of the nucleus 234 90

Th →

234 90

Th + 00 γ

– The γ ray is often emitted in conjunction with other particles Ch 20.13 of 64

Gamma (γ) Ray Emission • The γ ray has low ionizing potential because it is uncharged, causing low ionization in target atoms • It has the highest penetrating potential because it interacts with matter very weakly, stopped by tens of cm of lead or thick slabs of concrete • Causes low damage external and internal to the body in low doses (may pass through the body entirely)

Ch 20.14 of 64

Positron Emission • A positron is the antiparticle of the electron – Same properties but opposite charge – When two antiparticles meet they annihilate each other and release one or more γ rays

• A positron is produced when a proton decays Proton → Neutron + Positron • The positron is represented

Positron :

0 +1

e

Use the symbol e to avoid confusion with proton Ch 20.15 of 64

Positron Emission • When a positron is emitted, a proton is converted to a neutron in the nucleus so the atomic number decreases 30 15

P→

30 14

Si + +10e

• The identity of the element is changed • Positrons have similar ionizing and penetrating potential to β particles

Ch 20.16 of 64

Electron Capture • Unlike the the decay process described so far, electron capture involves absorption of a particle • A core electron is absorbed into the nucleus Proton + Electron → Neutron • Electron capture can be represented

p + −10e → 01n

1 1

• For example 92 44

Ru + −10e →

92 43

Tc

Z decreases by one because a proton is converted to a neutron Ch 20.17 of 64

Ch 20.18 of 64

20.4 Predicting Radioactive Decay • It is not easy to predict what kind of radioactive decay an atom will undergo (if any) • However, the larger the number of protons relative to neutrons, the more unstable the nucleus – Protons repel each other

• What holds the nucleus together? • The attractive short range nuclear strong force overcomes proton-proton repulsions • Neutrons attract other protons and neutrons through the strong force and stabilize the nucleus because they add no repulsive forces Ch 20.19 of 64

Predicting Radioactive Decay • The stability of a nucleus depends on N / Z, the neutron-to-proton ratio which increases with Z – Stable light elements have N / Z ≈ 1 – Stable heavy elements have N / Z ≈1.5 Radioactive isotopes

Stable isotopes Ch 20.20 of 64

The ‘Valley Of Stability’ • The ‘valley of stability’ shows that nuclei cluster close to an ideal N / Z ratio that varies with Z • If N / Z is too high (above the valley of stability), nuclei have too many neutrons – They tend to convert neutrons to protons through β decay

• If N / Z is too low (below the valley of stability), nuclei have too many protons – They tend to convert protons to neutrons by positron emission or electron capture (or less frequently by α emission) Ch 20.21 of 64

The ‘Valley Of Stability’ • Is Mo-102 more likely to undergo β decay or positron emission? • Mo-102 has 42 protons and 60 neutrons so N / Z = 1.43 • At this Z, N / Z ≈ 1.3 so the nuclide has too many neutrons • It will likely undergo beta decay, converting neutrons to protons

Ch 20.22 of 64

Magic Numbers • In addition to N / Z, the actual number of neutrons and protons affects stability Z

N

Number of Nuclides

Even

Even

157

Even

Odd

53

Odd

Even

50

Odd

Odd

5

Most nuclides have an even number of neutrons and protons

Few nuclides have odd number of neutrons and protons Ch 20.23 of 64

Magic Numbers • Just as electrons occupy energy levels in an atom, nucleons (protons and neutrons) occupy energy levels in the nucleus • We already know certain numbers of electrons have special stability (2, 10, 18, 36, 54…) • So certain numbers of protons or neutrons have special stability (N or Z = 2, 8, 20, 28, 50, 82, 126) • These are called magic numbers

Ch 20.24 of 64

Radioactive Decay Series • All atoms with Z > 83 (Bi) are radioactive – No stable isotopes

• All decay by one or more α or β (sometimes γ) emission steps • The heaviest natural isotope is U-238 (Z = 92) 238 92

U→

234 90

Th + 42 He

α decay

• But the daughter Th-234 is radioactive too 234 90

Th →

Pa + −10e

234 91

β decay Ch 20.25 of 64

Radioactive Decay Series • Radioactive decay occurs until a stable isotope is reached (where Z ≤ 83) • The steps are shown in a radioactive decay series – N versus Z

• The one for U-238 ends at Pb-206 Ch 20.26 of 64

20.5 Detecting Radioactivity • The simplest detectors are film-badge dosimeters • A piece of light-tight photographic film becomes darker when exposed to radiation – They are cumulative detectors that continue to darken with each exposure – They only detect radiation exposure when processed

Film-badge dosimeter

Ch 20.27 of 64

Detecting Radioactivity • When ionizing radiation enters a Geiger-Müller tube, Ar ions are created and collected A real time reading is obtained

A ‘click’ is an ion created by a radioactive particle Ch 20.28 of 64

20.6 Kinetics of Radioactive Decay • Radioactivity is a natural part of our environment – Our food is naturally radioactive – The human body is naturally radioactive

• The Earth is 4.6 billion years old but radioactivity persists • Why? – Some elements have very long time for decay – New radioactive elements are being created by cosmic ray bombardment from space

Ch 20.29 of 64

Kinetics of Radioactive Decay • All radioactive decay is first order Rate = k·N where N is the number of radioactive nuclei and k is the rate constant • Most often, the rate constant is converted to a halflife t1/2 in the usual way

t1/ 2

0.693 = k

Review chapter 13.3

• Elements with long half-lives decay slowly Ch 20.30 of 64

Kinetics of Radioactive Decay • To calculate how many radioactive nuclei remain after a given period of time, Nt, we use the integrated first-order rate law

Nt Ln = −k·t N0 Initial number of radioactive nuclei

Two nuclides with the same Z may have very different t1/2 1/2 Ch 20.31 of 64

Kinetics of Radioactive Decay • What mass of C-14 (t1/2 = 5730 years) remains after 10,000 years if the initial mass is 145 mg? • Using the integrated rate law

0.693 k= = 1.209x10 −4 / yr t1/ 2 Nt Ln = −k·t N0

Nt = N0 ·e

−k·t

so

Nt = e −k·t N0

= 145 mg × e

Note mg is proportional to N

−1.206x10−4 / yr×10,000 yr

= 43.3 mg Ch 20.32 of 64

Radiocarbon Dating • The predictable way in which radioactive elements decay means they can be used to determine age • Radioactive C-14 is made in the upper atmosphere by neutron bombardment 14 7

N + 01n →

14 6

C + 11H

• It decays by β decay according to 14 6

C → 147 N + −10e

t1/ 2 = 5,730 years

Ch 20.33 of 64

Radiocarbon Dating • Through photosynthesis, C-14 is incorporated into plants and animals and a constant C-14 to C-12 ratio results • When plants or animals die, new C-14 is not incorporated and so the amount decays • By radiocarbon dating, the ratio of C-14 to C-12 can be used to measure the time since death – After one half-life, the C-14 to C-12 ratio will be 50% of its original value – After two half-lives, the C-14 to C-12 ratio will be 25% of its original value Ch 20.34 of 64

Radiocarbon Dating • A scroll has a C-14 (k = 1.209x10-4 /yr) decay rate of 9.05 emission events per gram per minute. If a new scroll has a decay rate of 12.04 emission events per gram per minute, how old is the scroll?

Nt Ln = −k·t N0

so

Nt Ln N0 =t −k

 9.05 / g·min  Ln   12.04 / g·min  t= = 2360 yr −4 −1.209x10 / yr

Note decay rate proportional to N for first order

Ch 20.35 of 64

20.7 Nuclear Fission • In the 1930s Enrico Fermi tried to create a new element (Z = 93) by bombarding U with neutrons 238 92

U + 01n →

239 92

U→

239 93

?+ −10e

β decay

• Fermi failed to confirm the presence of element 93 • Meitner, Strassman and Hahn repeated the experiment but found products much lighter than U • They had observed nuclear fission, the breaking apart of a nucleus Ch 20.36 of 64

Nuclear Fission Balanced?

• The neutron-induced fission of U is 235 92

U + 01n →

140 56

Ba +

93 36

Kr + 3 01n + energy

• Only the U-235 nuclide undergoes fission – Natural U is only 1% U-235; most abundant is U-238

• Notice that 3 neutrons are produced – These can start fission in neighboring U-235 atoms

• Samples rich in U-235 can undergo a selfamplifying chain reaction – Nuclear chain reactions release large amounts of energy Ch 20.37 of 64

Nuclear Chain Reactions Energy is released at each step To sustain the chain reaction, samples must be enriched in U-235

Ch 20.38 of 64

Critical Mass

The ‘demon core’

• A ‘runaway’ condition exists if enough neutrons strike enough U-235 atoms – If the mass is small, neutrons escape from the surface without causing enough chain reactions (subcritical) – If the mass is large, neutrons remain inside and cause runaway (supercritical)

• Critical mass of U-235 is ~52 kg (17 cm diameter)

Pu sphere surrounded by neutron-reflective WC blocks goes supercritical if blocks are added Ch 20.39 of 64

Atomic Bombs • Fearing the Nazis were developing their own atomic bomb, the Manhattan project developed the first US atomic bombs • Two were dropped on Japan in 1945 killing ~200,000 people

16 ms after detonation, ~200 m wide, ~10x1066 K

First atomic bomb test in New Mexico (Trinity test) in 1945 had the power of 18,000 tons of TNT Ch 20.40 of 64

Nuclear Power • If the energy of nuclear fission is released more slowly, it can be used to generate electricity • Most power plants work the same way: heat produced by fuel is used to make steam that spins electric generators (turbines) – 1 kg of U produces as much heat as 40,000 kg of coal – Fission produces no atmospheric pollution (CO2, NOx, Hg, SOx)

• 20% of US power is generated by nuclear fission • Nuclear power plants cannot become bombs because the U is only enriched 3.5% Ch 20.41 of 64

Neutronabsorbing control rods drop between fuel rods to slow fission

Nuclear Power Water circulates around core

Concrete shell contains core

Control rods drop (scram) when power fails Ch 20.42 of 64

Nuclear Power Disadvantages • Accidents may spread radioactivity – Chernobyl (1986)

• Waste disposal – Products are produced in small quantities but they are intensely radioactive with long half-lives – All US nuclear waste is stored at the nuclear power plant – Central storage being developed at Yucca Mountain, NV

Chernobyl reactor core Ch 20.43 of 64

20.8 Converting Mass To Energy • When nuclear particles (nucleons) form a nucleus, energy is released (nucleus is more stable) • Think about forming a He-4 nucleus from its components proton

neutron

2 11H + 2 01n → 42 He • Mass reactants = 2 p+ + 2 n0 = (2 x 1.00783 amu) + (2 x 1.00866 amu) = 4.03298 amu Why • Mass products = 4.00260 amu different? Ch 20.44 of 64

Converting Mass To Energy • A He atom has less mass than its components? • This is called the mass defect • The mass is converted to energy when the nucleus forms according to E = m·c2 • This energy is called the nuclear binding energy; the energy required to break apart the nucleus • Nuclear chemists report the binding energy in MeV (megaelectronvolts) with 1 amu ≡ 931.5 MeV Ch 20.45 of 64

Converting Mass To Energy • For the He atom, the mass defect = 0.03038 amu or 28.30 MeV – 1 MeV = 9.65x1010 J/mol – 28.30 MeV = 2.73x1012 J/mol He

• The binding energy varies for each nuclide so to compare we calculate the binding energy per nucleon 28.30 MeV Binding energy per nucleon = = 7.08 MeV 4 nucleons 2 protons 2 neutrons Ch 20.46 of 64

Nuclear Binding Energy

When light atoms fuse, binding energy increases and energy is released

When heavy atoms break into lighter atoms, binding energy increases and energy is released Ch 20.47 of 64

Nuclear Fission • How much energy is released from U-235 fission? 235 1 U + 92 0n

→

142 54 Xe

+

90 38 Sr +

4 01n

• We calculate ∆m = -0.2055 g/mol • And ∆E = ∆m·c2 = -1.847x1010 kJ/mol • Or -7.86x107 kJ for each 1 g of U-235 – Enough energy to heat 250 million liters of water from 25 °C to 100 °C – It would take 1650 kg of octane to release the same amount of energy! Ch 20.48 of 64

20.9 Nuclear Fusion • When a heavy atom splits (nuclear fission) the binding energy increases and energy is released • When two light atoms fuse (nuclear fusion) the binding energy increases and energy is released – Fusion reactions power the sun – Fusion reactions take place in ‘hydrogen bombs’ 3 4 1 H + H → He + 2 0n { {1 2 1

deuterium

tritium

• Very high temperatures are required Ch 20.49 of 64

Nuclear Fusion • Consider the mass difference in a fusion reaction 2 3 H + 2 He {1 { 2.01345 amu 3.01493 amu

→

4 1 He + H {2 {1 4.00150 amu 1.00728 amu

• The change in mass is ∆m

∆m = (4.00150 + 1.00728) − (2.01345 + 3.01493) amu = −0.01960 amu = −0.01960 g/ mol = −1.960x10−5 kg/ mol Ch 20.50 of 64

Nuclear Fusion • Let’s convert this to energy

∆E = ∆m ⋅ c 2 = −1.960x10 −5 kg / mol × (2.998x108 m / s)2 = −1.76x10 kg ⋅ m / s ⋅ mol 12

2

2

= −1.76x1012 J / mol • 1.76 billion kJ of energy / mol!

Million times more than typical chemical reactions

Ten times more than typical fission reactions Ch 20.51 of 64

Nuclear Fusion • Very high temperatures are required (>10,000 K) for fusing two positive nuclei • Atoms must be contained by magnetic fields or lasers • Fusion has been achieved for short times but much more energy required than produced

The ‘Tokamak’ fusion reactor is a ‘wall-less’ container Ch 20.52 of 64

20.11 Effects of Radiation on Life When α and β particles strike living cells considerable damage may occur 1. Acute damage • Large amount of radiation in a short period of time

•

– Rapidly dividing cells are most susceptible (intestinal, reproductive and immune cells)

•

Large numbers of ions created within the cell that react with and destroy important cell molecules leading to cell death

Ch 20.53 of 64

Effects of Radiation on Life 2. Chronic damage • Large amount of radiation over a long period of time • DNA is damaged at a faster rate than it can be repaired in the cell • Cell may die or grow abnormally (cancer) • If DNA of reproductive cells is damaged, it may be passed to offspring (genetic mutations) – Genetic diseases in offspring may result

Ch 20.54 of 64

Measuring Radiation Exposure • • •

Major unit of radioactivity is the Curie (Ci) where 1 Ci = 3.7x1010 decay events / s But 1 Ci exposure to α particles will do more damage than 1 Ci exposure to β particles It is better to measure amount of energy deposited in the body 1 Gray (Gy) = 1 J / kg body tissue 1 rad = 0.01 Gy = 0.01 J / kg body tissue

•

But this does not account for the type of radiation Ch 20.55 of 64

Measuring Radiation Exposure •

The rad is multiplied by the relative biological effectiveness (RBE) factor to produce the rem unit 1 rem = 1 rad x RBE – The RBE for α particles is much higher than γ rays and both depend on their energy

• •

Average person receives ~360 mrem per year Measurable physiological effects occur at ~20 rem

Ch 20.56 of 64

Ch 20.57 of 64

Measuring Radiation Exposure

•

Professions with particularly high radiation risks are health workers, flight crew, underground miners

Ch 20.58 of 64

20.12 Radiation in Medicine •

Diagnosis in medicine is improved by using radiotracers, radioactive nuclides of elements commonly found in the body – Radiotracers are easily detected – Radiotracers have identical chemistry to their nonradioactive counterparts

•

•

Radioactive iodine-131 is taken into the thyroid gland with regular iodine but can be detected so the uptake rate of iodine quantified Difference elements are concentrated in different parts of the body Ch 20.59 of 64

Tc-99 is concentrated in bone during a ‘bone scan’

Ch 20.60 of 64

Positron Emission Tomography •

F-18 labeled glucose is injected into the bloodstream – The F-18 decays by positron emission

•

•

The emitted positron and nearby electrons collide, annihilate each other and produce 2 γ rays in opposite directions Detectors pinpoint when the γ rays originated

A PET scan shows area where brain activity (glucose metabolism) is highest Ch 20.61 of 64

Radiotherapy •

•

Radiation is particularly effective at killing dividing cells and is used in cancer treatment Focused γ rays are moved in a circle around the patient – Maximizes tumor and minimizes body exposure

•

Patients often develop radiation sickness symptoms

Each dose ~100 rem or 1% increase in cancer risk Ch 20.62 of 64

Radiotherapy • • •

How can radiation both cause and cure cancer? Answer lies in risk management If a person has a 95% chance of dying of cancer versus a 1% increased risk of cancer for each treatment, the risk is acceptable

Ch 20.63 of 64

Other Uses of Radiation •

Radiation kills bacteria and viruses – Sterilize surgical instruments – Sterilize food (this does not make the food radioactive)

•

Control insect populations by sterilizing males and releasing them

‘Radura’ logo identifies food treated with radiation Ch 20.64 of 64