Atomic & Nuclear Chemistry

Monday 10/29/12Prep:

1.Find geiger counter & samples

Class:

1.Coop Quiz – A Mole Conversion & An Avg Atomic Mass Problem

2.Isotopes (Quick Review) Radiation Demos

3.Alpha, Beta, Gamma Radiation

4.Fission vs Fusion

5.Nuclear Equations – Alpha, Beta

Asmt: Write Nuclear Equations for the Uranium Decay Series

Vocabulary 2B (Define in your notebook)

Isotope nuclear reactorRadioisotope nuclear weaponRadioactivity half lifeRadiation nuclear equationFission positronFusion radiocarbon datingRadioactive decay critical massAlpha particle nuclear bombardmentBeta particle curieGamma ray remNuclear chain reaction plasmaDosimeter transmutation

Isotopes• Atoms of a single element have the same

number of protons but may differ in neutrons.• Example 1: Carbon-12 vs Carbon-14• Example 2: Uranium-238 vs Uranium-235• Some isotopes are stable while others are

unstable and radioactive.• The STRONG NUCLEAR FORCE acts between

protons & neutrons to hold them together. However protons will repel each other with their mutual positive charge.

Carbon Isotopes

Isotope Half – life

Carbon – 9 0.1265 s

Carbon – 10 19.2 s

Carbon – 11 20.38 min

Carbon – 12 Stable

Carbon – 13 Stable

Carbon – 14 5715 y

Carbon – 15 2.449 s

Carbon – 16 0.75 s

• How long does it take 400 g of each isotope to decay to less than 1 mg?

Geiger Counter DemosSample Counts per

MinuteReason

Humans

NaCl vs KCl

Smoke Detector

Old Fashioned Lantern Mantle

Old Glow in the Dark Clock

Uranium Ore

Becquerel/Curries• Becquerel – First observed photographic

evidence of radioactivity

• Curie – Discovered radioactive elements of radium and polonium

• Curie – made gift of 1 gram of Radium to President Truman to help fight cancer.

Rutherford’s Discovery of Alpha, Beta & Gamma Radiation

Rutherford & RadiationDiscovered alpha, beta & gamma radiation using a magnetic field.

Radioactivity

Blocked by 1ft of concrete or few inches of lead

Sheet metal

Blocked by paper

Penetration

High energy photon

1 electron2 protons,

2 neutrons

Composition

0 (movie)-1+2Charge

0 amu1/1837 amu

4 amuMass

Symbol

GammaBetaAlphaTypes of

Radiation

Radioactivity

• Alpha – largeRelatively slow

• Beta – much smallerRelative fast

• Gamma – no massPure energy

Travels at the

Speed of light

Alpha Radiation

• The unstable nucleus simultaneously ejects two neutrons and two protons, which correspond to a helium nucleus. 

• The emission of gamma photons is a secondary reaction that occurs a few thousandths of a second after the disintegration.

Alpha Emission

263 Sg106

4 He

2+

259 Rf104

Beta Emission

• The unstable nucleus ejects an electron.• The emission of gamma photons is a secondary

reaction that occurs a few thousandths of a second after the disintegration

Beta Emission

14 C 6

0 e-1

+14 N 7

+

A neutron disintegrates, ejects an electron and another proton is left over. A gamma ray is also released.

Gamma Radiation involves high energy waves that don’t change

the identity of the nucleus

Nuclear Reactions• Radioactivity results from changes in

atomic nuclei.

• Fission – splitting of a large nucleus into smaller pieces releases energy.

• Fusion – small nuclei join to make a larger nucleus and release energy.

• Energy is released when a small amount of mass converts to energy as E = mc2.

• The atomic masses of isotopes deviate from whole numbers due to loss of mass from energy released as they fused.

Fusion of Hydrogen Isotopes

• At high temperatures and pressures, 2 nuclei may collide and form a bigger nucleus.

• This example produces helium and a stray neutron.

• Stars are fueled by the energy released by fusion which also builds atoms of increasing sizes in their cores.

Fission of Uranium

• A neutron splits the nucleus.

• The fragments include:– 2 different smaller

atoms,– 3 more neutrons.

• The 3 neutrons can split more atoms.

• If every fission splits 3 more atoms, the reaction will multiply out of control!

Nuclear Chain

Reaction

Nuclear Equations

• Styrofoam Demos

• Alpha Decay – releases a helium nucleus.

• Beta Decay – a neutron converts to a proton and releases an electron.

• Assignment: Uranium Decay Series

Nuclear Warheads

Chernobyl Nuclear Disaster

Nuclear Equations• Alpha () Decay – releases 2 protons & 2

neutrons - a helium nucleus.

• Beta () Decay – a neutron converts to a proton and releases an electron.

4 He2

0 e-1

Uranium Decay Series

Uranium Decay Series

• U238 alpha - HL 4.468e9y

• Th234 beta – HL 24.10d• Pa234 beta – HL 6.70h• U234 alpha – HL

245,500y• Th230 alpha – HL

75,380y• Ra226 alpha – HL1600y• Rn222 alpha – HL

3.8325d

• Po218 alpha – HL 3.10m• Pb214 beta – HL 26.8m• Bi214 beta – HL 19.9m• Po214 alpha – HL 164.3

s• Pb210 beta – HL 22.6y• Bi210 beta – HL 138d• Po210 alpha – HL

4.199m• Pb206 Stable!

Nuclear Equations• Uranium 238 does alpha decay:

– Mass numbers balance on both sides.– Atomic numbers balance on both sides

• Thorium 234 then does beta decay:

4 He2

238 U92

234 Th90

+

234 Th90

0 e-1

234 Pa 91

+

Nuclear Equations Problems

1. U–238 does alpha decay in nuclear reactors.

2. Am-241 does alpha decay in smoke alarms.

3. Tc-98 does beta decay in medical exams.

4. C–14 does beta decay in carbon dating.

5. The Curies used Ra-226 which does alpha decay.

6. Co–60 does beta decay in food irradiation.

7. Th-232 does alpha decay in camp lanterns.

8. P-35 does beta decay in DNA studies

(Place isotopes activity in Outbox)

Nuclear Equations Quiz

1.Write the nuclear equation for the alpha decay of Iodine 131.

2.Write the nuclear equation for the beta decay of cobalt 60.

½ Lives Activity• Obtain a set of “radioactive” blocks. Notice that each one has

a mark on one side – either a, b or g.

• Roll the collection of blocks onto your table. Each time you roll, remove any blocks that come up , or .

• Count and record the remaining blocks. Roll the remaining blocks repeatedly 20 times and complete the chart below.

• Enter your group data into the excel file.

• Make graphs of Time(minutes) Remaining Atoms for both individual & class averages. **Use “exponential” rather than “linear” trendlines.

Roll

(minutes)

Remaining Atoms

Class Average

½ Lives Activity Questions

1. How do your lab pair results compare with the class average results?

2. Use the class average results and compute the 1st ½ life, 2nd ½ life, average ½ life.

3. What importance do ½ lives have to society?

(dating, medical uses, wastes)

½ Lives

• Each radio-isotope decays at a characteristic rate.

• The decay rate is determined by the time that it takes for ½ of the radio-isotope nuclei to break down by fission.

• Each ½ life reduces the remaining number of radioactive atoms by ½.

• The number remaining approaches but never reaches zero.

• Example: Iodine 131 has a ½ life of 8 days. How much of 1.00 gram sample would remain after 24 days?

½ Lives of Isotopes

Isotope Half – life Radiation emitted

Carbon – 14 5715 y Beta

Potassium – 40 1.3e9 y Beta, gamma

Radon – 222 3.8 d Alpha, gamma

Radium – 226 1600 y Alpha, gamma

Iodine – 131 8.07 d Beta, gamma

Thorium – 234 24 d Beta, gamma

Uranium – 235 7.0e8 y Alpha, gamma

Uranium – 238 4.5e9 y Alpha, gamma

Solving ½ Life Problems

Masses:• STARTING

MASS• Divided in ½ the

# of half lives.• ending mass

Times:• Time for 1 half

life (HL)• Total time

elapsed (T)• T = HL*(#)• HL = T/#• # = T/HL

# of halflives

½ Life Example 1 – Iodine 131 has a ½ life of 8 days. How much of 1.00 gram sample would

remain after 24 days?

Times:

½ life = 8 days

Total = 24 days

24days / 8days

= 3 half lives

Masses:

Start = 1.00 g

End = ?unknown

If 3 half lives occur,

divide start by 2 3-times

1.00 g / 2 / 2 / 2

= .125 g

½ Life Example 2: A 8.8mg sample of chromium-55 is analyzed after 15 min and found to contain 1.1mg remaining. What is the ½ life

of Cr55?

Masses:

Start = 8.8 mg

End = 1.1mg

Divide 4.4 by 2 until reaching 1.1.

8.8/2 = 4.4

4.4/2 = 2.2

2.2/2 = 1.1

3 – ½ lives occurred

Times:

Total = 15 min

½ life = ? Unknown

Divide 15 min by 3 – ½ lives

15min/3 HL

= 5 min/1HL

5 min = 1 – ½ life

½ Life Problems1. If you have $1 million dollars and every 2

seconds it decreases by 1/2, how long will it take until you are penniless?

2. If a sample of a fossil mammoth has 1/8th the amount of carbon 14 as it would today, how old must the fossil be? (1/2L C14 = 5715 years.

3. If a rock contained 1.2 g of potassium 40 when it formed, how many grams remain after 4 billion years. (1/2L K40 = 1.3e9 y)

Asmt: P780 #1&2, P803 #24&25

More ½ Life Problems4. If a sample of radioactive isotope has a half-life of 1 year,

how much of the original sample will be left at the end of the second year? The third year? The fourth year?

5. The isotope cesium-137, which has a half-life of 30 years, is a product of nuclear power plants. How long will it take for this isotope to decay to about one-sixteenth its original amount?

6. Iodine-131 has a half-life of 8 days. What fraction of the original sample would remain at the end of 32 days?

7. The half-life of chromium-51 is 28 days. If the sample contained 510 grams, how much chromium would remain after 56 days? How much would remain after 1 year?

½ Lives Quiz

1. A sample of a radioactive isotope with an original mass of 8.00g is observed for 30 days. After that time, 0.25g of the isotope remains. What is its half-life?

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

Health Physics Society• http://hps.org/publicinformation/ate/q754.html

• Q:What are some health effects of the element uranium?

• A:The toxicity of uranium has been under study for over 50 years, including life-span studies in small animals. Depleted uranium and natural uranium both consist primarily of the uranium isotope 238U. They are only very weakly radioactive and are not hazardous radioactive toxicants, but uranium is a weak chemical poison that can seriously damage the kidneys at high blood concentrations. Virtually all of the observed or expected effects are from nephrotoxicity associated with deposition in the kidney tubules and glomeruli damage at high blood concentrations of uranium. The ionizing radiation doses from depleted and natural uranium are very small compared to potential toxic effects from uranium ions in the body (primarily damage to kidney tubules).