topic 12 nuclear chemistry. what is radioactivity? isotopes of many atoms are unstable (have extra...

Topic 12

Nuclear

Chemistry

Topic 12

Nuclear

Chemistry

What is Radioactivity?What is Radioactivity? What is Radioactivity?What is Radioactivity? Isotopes of many atoms are unstable (have extra energy). These are called radioisotopes.

Radioisotopes emit this extra energy (radiation), to become more stable, by splitting up (fission).

When a radioisotope splits up new atoms with different atomic and mass numbers are formed. Radioisotopes that emit radiation are radioactive. The emission process is called radioactivity.

Radioactivity is a totally random (spontaneous) process. It cannot be predicted as far as which atoms will decay; however, the overall half-life of radioisotopes is well known.

Historical BackgroundHistorical BackgroundHistorical BackgroundHistorical Background Wilhelm Roentgen – discovered x-rays Henri Becquerel – found Uranium ores emit

radiation that resembled x-rays Marie & Pierre Curie – isolated two new

elements (Po & Ra) from uranium ores Ernest Rutherford - showed there were 3 types

of radiation , , and *The Curies’ & Becquerel were jointly awarded

the Nobel prize in Physics for discovering Radioactivity in 1903

*In 1911 Marie Curie won a second Nobel prize in Chemistry for isolation of pure radium metal

Nuclear Chemsitry - Nuclear Chemsitry - Nuclear Chemsitry - Nuclear Chemsitry - nucleus

neutrons

electrons

unstable

Size of the nucleus Ratio of protons to neutrons

Nuclear Belt of StabilityNuclear Belt of StabilityNuclear Belt of StabilityNuclear Belt of Stability Protons repel each other-

The higher the atomic number is, the greater the repulsion among protons, making the nucleus unstable.

Neutrons help to stabilize the nucleus

As the number of protons increases, the number of neutrons needed to keep the nucleus stable increases

Stable atoms have a ratio of neutrons to protons that falls in the belt of stability

6

Only certain combinations Only certain combinations of protons and neutrons are of protons and neutrons are

stable.stable.

Only certain combinations Only certain combinations of protons and neutrons are of protons and neutrons are

stable.stable.A nuclide falling outside the “belt of stability” are radioactive. They spontaneously decay to form another element.

There are no stable isotopes of elements above atomic

number 83.

Nuclear Belt of StabilityNuclear Belt of StabilityNuclear Belt of StabilityNuclear Belt of Stability

Radioactive Particles (emissions)

Radioactive Particles (emissions)

Radiation Particle

Mass(Amu)

Charge Symbol or Notation

Penetrating power

Alpha particle

4 2+ 4 He2 or (α) 

Low: Stopped by paper or cm of air

Beta particle

0 1- 0 e-1 or (B-)

Moderate: Stopped by 3 mm aluminum

Positron  

0 1+ 0 e+1 or (B+)

Moderate: Virtually zero range

Gamma ray

0 0 

( γ ) High: Stopped by cm of PB or meters of concrete

• A radioisotope can lose energy by emitting three different types of radiation; refer to Table O

9

Alpha particles have the lowest Alpha particles have the lowest

penetrating power, gamma the highestpenetrating power, gamma the highest..Alpha particles have the lowest Alpha particles have the lowest

penetrating power, gamma the highestpenetrating power, gamma the highest..

Alpha particles

won’t pass through paper.

Alpha particles have the highest ionizing power. They knock off electrons and leave a trail of ions

as they pass through the air.

Separation of Radioactive of Radioactive EmissionsEmissions

Separation of Radioactive of Radioactive EmissionsEmissions

ELECTROMAGNETIC FIELD•Opposites charges attract

Types of Nuclear Types of Nuclear ReactionsReactions

Types of Nuclear Types of Nuclear ReactionsReactions

A. Transmutations –when one atom changes into another more stable atomWhen a nucleus emits radiation, the # of protons changes, and so the element changesTransmutations can be natural (spontaneous) or artificial (induced)

Natural just happens (ordinary decay of an unstable isotope). Radioisotopes that spontaneously decay are listed on reference table N.

Induced means to bombard nuclei with high NRG particles (nuclear bullets) to make it unstable (like a nuclear reactor).

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Each radioactive isotope has a Each radioactive isotope has a specific mode and rate of decay specific mode and rate of decay

(half-life). (half-life).

Each radioactive isotope has a Each radioactive isotope has a specific mode and rate of decay specific mode and rate of decay

(half-life). (half-life).

This decay can be written in the form of a nuclear equation. the MASS AND CHARGE must be

balanced on both sides of the equation nuclides decay to achieve a stable

ratio of neutrons to protons

He Th U 42

23490

23892

e Xe I 0-1

13154

13153

e Ar K 01

3818

3819

Pd e Ag 10646

0-1

10647

Writing Nuclear Writing Nuclear EquationsEquations

Writing Nuclear Writing Nuclear EquationsEquations

Use Table N & OMass and Charge must be

balanced on both sides of the equation

e Xe I 0-1

13154

13153

Changes in Mass Number and Atomic Changes in Mass Number and Atomic Number That Occur When Radioactive Number That Occur When Radioactive

Elements DecayElements Decay

Changes in Mass Number and Atomic Changes in Mass Number and Atomic Number That Occur When Radioactive Number That Occur When Radioactive

Elements DecayElements Decay

1. Natural Decay: spontaneous

1. Natural Decay: spontaneous

Decay mode(see Table N)

PARENT NUCLIDE → DAUGHTER NUCLIDE plus an emission

α 238

92 U → _____________ + ____________

- 234

90 Th → _____________ + ____________

+ 37

19 K → _____________ + ____________

Decay ModesDecay Modes

He Th U 42

23490

23892

parentnuclide

daughternuclide

alphaparticle

Alpha emission: mass decreases by four, atomic number decreases by two.

238U undergoes alpha decay

The total mass on the left must equal the total mass on the right (238 = 4 + 234)

The total charge on the left must equal the total charge on the right (92 = 2 + 90)

Decay examples Decay examples continuedcontinued

Decay examples Decay examples continuedcontinued

Beta Emission: mass remains the same, atomic number increases by one.

e Pa Th 0-1

23491

23490

electronThe total mass on the left must equal the total mass on

the right (234 = 0 + 234)The total charge on the left must equal the total charge

on the right (90 = -1 + 91)

Decay examples Decay examples continuedcontinued

Decay examples Decay examples continuedcontinued

Positron Emission: mass remains the same, atomic number decreases by one.

e Ar K 01

3718

3719

positron

The total of the mass numbers on the left must equal the total on the right (37 = 0 + 37)

The total charge on the left must equal the total charge on the right (19 = 1 + 18)

2. Artificial Decay 2. Artificial Decay 2. Artificial Decay 2. Artificial Decay particle accelerators give charged particles enough energy to

overcome electrostatic forces & penetrate the nucleus of an atom There will be 2 reactants (a target nuclei and a particle “bullet”) Ex 1.

Ex 2

X is _______

Remember…Mass & Charge must be equal

F He N 189

42

147

Nuclear bulletNuclear bullet

n XHe Al 10

42

2713

neutron, beta, or alpha

particle

Let’s practice……RB pg 217-220 #’s 1-15; guide pg 6

4. Half-Life4. Half-Life4. Half-Life4. Half-Life The amount of time that it takes for

1/2 a sample to decay.

radioactivity of a sample deceases over time but never reaches zero

each time a decay happens, one more of the original radioactive nuclei has disappeared

as the unstable nuclei steadily disappear, the activity of the whole also decreases. Therefore, the older the sample, the less radiation it will emit (shorter ½ life, less stable it is)

Half-Life

Each radioactive isotope has a particular rate at which it decays. See reference Table N for sample radioisotopes

The half-life or rate of decay is CONSTANT (STAYS THE SAME) regardless of the sample size, or other physical conditions (temperature or pressure).

Decay of 20.0 mg of 15O. What remains after 3 half-lives? After 5 half-lives?

Half Life CalculationsHalf Life CalculationsHalf Life CalculationsHalf Life CalculationsDoing calculations, use chart or arrows and always

start time with zero.Number of

½ lives%

remaining)Fraction

remainingDecimal

remaining0 100 1 1

1 50 1/2 .5

2 25 ¼ .25

3 12.5 1/8 .125

4 6.25 1/16 .0625

5 3.125 1/32 .03125

Time of half life Amount decaying

0  

Practice Practice problemsproblems Practice Practice problemsproblems Ex 1) How much phosprous-32 (P-32) is left after 3 half-lives if the

original sample contained 200 mg?

Time ofhalf-life

Amount decaying

0 200 

 1  100

2  50

 3  25

As you go down, mass amount decreases by

1/2

Using Arrows (represent a ½ life passing):

0 1 2 3

200mg 100 mg 50 mg 25 mg

Using Chart: start time with zero

Half-life problemsHalf-life problemsHalf-life problemsHalf-life problems

Use this formula (½)t/T from Table T if the question states “what fraction or percentage”, otherwise             

use   t/T = # of half-life cycles from Table T

Example #1Example #1Example #1Example #1

What mass of iodine-131 remains 32 days after a 100-g sample of this  isotope is obtained?

Use t/T = # of half-life cycles and Reference table N to find T, then find mass

32 d/8.07 d =4 half-life cycles Cut the 100 g in half 4 times100502512.56.25g is the answer!

Example #2Example #2Example #2Example #2

The starting mass of a radioactive isotope is 20.0 g. The half-life period of this isotope is 2 days. What percentage of the original amount remains after 14 days?

Use this formula (½)t/T

(½)14d/2d = (½)7 ½ x ½ x ½ x ½ x ½ x ½ x ½ = 1/128 x100 = 0.78% is the answer!

The total amount of matter and energy cannot be destroyed.

The loss of mass in nuclear reactions represents a conversion of some matter into energy

The matter that has been converted into energy is called the mass defect

Part 2 – Nuclear Energy- Part 2 – Nuclear Energy- Conservation of matter to energyConservation of matter to energy

Part 2 – Nuclear Energy- Part 2 – Nuclear Energy- Conservation of matter to energyConservation of matter to energy

5. Nuclear Changes: Converting Matter to Energy (known as MASS DEFECT)

5. Nuclear Changes: Converting Matter to Energy (known as MASS DEFECT)

Energy released in a nuclear reaction comes from the fractional amount of mass being converted to energy

Energy released during nuclear reactions is much greater than the energy released during chemical reactions

The basics of nuclear energy – YouTube http://www.powerhouseanimation.com/PRG/PHYS025.swf

The basics of nuclear energy – YouTube http://www.powerhouseanimation.com/PRG/PHYS025.swf

a) Nuclear Fission: a) Nuclear Fission: splitting atomssplitting atoms a) Nuclear Fission: a) Nuclear Fission: splitting atomssplitting atoms bombarding the nucleus of a heavy atom with neutrons- this

splits the big atom into two smaller atoms and releasing some neutrons and a tremendous amount of energy

Used in power stations (NRG produced is used to heat water that drives steam turbines) the neutrons released in the reaction can cause additional chain reactions

(diagram of reaction) an uncontrolled chain reaction results in a nuclear explosion (atomic bomb) a controlled chain reaction can be used as a source of energy (nuclear reactor)

Fission = splitting atomsFission = splitting atomsFission = splitting atomsFission = splitting atoms

Figure 19.6:Figure 19.6: Diagram of a nuclear power Diagram of a nuclear power plant.plant.

Nuclear Power PlantNuclear Power Plant

Figure 19.6:Figure 19.6: Diagram of a nuclear power Diagram of a nuclear power plant.plant.

Nuclear Power PlantNuclear Power Plant

Nuclear Fission & Nuclear Fission & POWERPOWER

fukushimafukushima

Nuclear Fission & Nuclear Fission & POWERPOWER

fukushimafukushima Currently about 103 nuclear Currently about 103 nuclear

power plants in the U.S. and power plants in the U.S. and

about 435 worldwide.about 435 worldwide.

NRC: List of Power Reactor NRC: List of Power Reactor

Units Nuclear Regulatory Units Nuclear Regulatory

CommiteeCommitee

17% of the world’s energy 17% of the world’s energy

comes from nuclear.comes from nuclear.

Nuclear power plants in Nuclear power plants in operation in the U.S. in operation in the U.S. in

2002.2002.

Nuclear power plants in Nuclear power plants in operation in the U.S. in operation in the U.S. in

2002.2002.

b) Nuclear Fusionb) Nuclear Fusionb) Nuclear Fusionb) Nuclear Fusion

Two light isotopes come together (combine) (fuse together) releasing a lot of energy

this happens naturally in stars (hydrogen fusing together to form helium).

 Ex 2: 3H + 3H → 4 He + 2 1 n + energy 2 2 2 0

 Ex 3: 3He + 1H → 4 He + 0 e + energy 2 1 2 1

Fission vs. FusionFission vs. FusionFission vs. FusionFission vs. Fusion

• Nuclear = 19 % of US electrical energy use.

danger of nuclear reactor meltdown (safety risks)

Reactor releases toxic wastes to the environment Can get into food chain-

Long-term storage of nuclear waste (½ life)!!

fuel is abundant no toxic waste (cleaner) High energy requirements- not

yet sustainable - in order for nuclei to combine they need enough energy to overcome the forces of repulsion between like charges

FISSION

FUSION

Nuclear WeaponsNuclear WeaponsNuclear WeaponsNuclear Weapons

Atomic Bomb chemical explosion is used to form a critical

mass of 235U or 239Pu fission develops into an uncontrolled chain

reaction

Hydrogen Bomb chemical explosion fission fusion fusion increases the fission rate more powerful than the atomic bomb

Uses of RadioisotopesUses of RadioisotopesUses of RadioisotopesUses of Radioisotopes

I

III

II

Radiation = mostly natural (82%)

BEIR V, National Academy

Press, 1990

Single and Repeated Exposures Have Single and Repeated Exposures Have Impact Impact

Single and Repeated Exposures Have Single and Repeated Exposures Have Impact Impact

the effects of nuclear radiation on the body can add up over time.

6. Uses of Radioactive 6. Uses of Radioactive MaterialsMaterials6. Uses of Radioactive 6. Uses of Radioactive MaterialsMaterials

1. Radioactive Dating: 1. Radioactive Dating: C-14 (carbon dating) and U-238/Pb-206 (mineral/rock dating)

2. Industrial 2. Industrial measurement (gamma rays) – to detect the thickness/strength of materials.

3. Sterilzation3. Sterilzation: kills/prevents bacteria and other disease causing organisms: Co-60 and Cs-137 and gamma radiation (food irradiationfood irradiation)

Fact..Fact..•Irradiated milk has a shelf life of 3 mo. without refrigeration.Irradiated milk has a shelf life of 3 mo. without refrigeration.• USDA has approved irradiation of meats and eggs.USDA has approved irradiation of meats and eggs.

4. Tracers in MedicineMedicine: QuicklyQuickly eliminated and have short short half-lives Treating disorders and disease I-131 I-131 (thyroid), Tc-99 (brain tumors)

5. RadiotherapyRadiotherapy – gamma rays to kill cancer cells or Ra-226 and Co-60 (cancer therapy)

6. Nuclear power Nuclear power – radioactive decay produces energy in the form heat which later helps to produce electricity Fissionable fuels: Pu-239 & U- 235Fissionable fuels: Pu-239 & U- 235

Radiation treatment using-rays from cobalt-60.

***Fact...Tc -99 is probably the most widely used radioisotope in medicine today; it is a decay product, of molybdenum-99.

7. Risks and Hazards of 7. Risks and Hazards of RadioactivityRadioactivity

7. Risks and Hazards of 7. Risks and Hazards of RadioactivityRadioactivity

1. Harms living cells Damages/destroys cellscells Lower doses give rise to mutant cells causing cancer cancer (ionizing molecules) Higher doses can cause radiation sicknessradiation sickness

2.   Radioactive wastewaste must be stored safely Burying radioactive wastes from nuclear power plants (Cs-137, N-16,

Kr-85, and Rn-222). Long half-livesLong half-lives

1. “NIMBY” syndrome- don’t bury the waste in my backyard. Where do we store these wastes so they don’t get into the food chain?

3. Nuclear Accidents Nuclear Accidents affect the environment for many years afterwards

Safety issues concerning nuclear power plants • Chernobyl accident in Ukraine 1986

(contaminated farmland and livestock in Western Europe and North America)

• Nuclear fallout damages people’s health causing cancer and perhaps death.

Effects of Radiation……Let’s watchhttp://www.kentchemistry.com/links/Nuclear/EffectsofRadiation.htm

Uses of RadioisotopesUses of RadioisotopesRead pp226-227 and fill in chart & complete Read pp226-227 and fill in chart & complete questions 48-57questions 48-57

Uses of RadioisotopesUses of RadioisotopesRead pp226-227 and fill in chart & complete Read pp226-227 and fill in chart & complete questions 48-57questions 48-57

Radioisotope Use

C-14

I-131

Co-60

Tc-99

Pu-239

Am-241

U-235

U-238

Characteristics of Radioisotopes use in nuclear medicine