Ch. 20: Radioactivity and Nuclear Chemistry

Dr. Namphol Sinkaset Chem 201: General Chemistry II

I. Chapter Outline

I. Introduction II. Types of Radioactivity III. The Valley of Stability IV. Radiometric Dating V. Nuclear Fission VI. Nuclear Fusion VII. Transmutation VIII. Radiation and Life

I. Introduction

• Antoine-Henri Becquerel discovered radioactivity accidentally while studying x-rays and phosphorescence (the “glow” in “glow in the dark”).

I. Introduction • Crystals of

potassium uranyl sulfate were used to try and prove that phosphorescence occurred with x-ray emission.

• His experiments involved sunlight, photographic plates, and a black cloth.

I. Introduction • Becquerel concluded that the uranium

caused the exposure and called the emissions uranic rays.

• Marie Curie studied uranic rays for her doctoral thesis and discovered they weren’t unique to uranium.

• She discovered 2 new elements that had the same emissions and renamed the phenomenon radioactivity.

I. Introduction • The Curies and

Becquerel won the Nobel Prize in physics for the discovery of radioactivity in 1903.

• Marie Curie also won the Nobel in chemistry in 1911 for discovering Ra and Po.

II. Types of Radioactivity • Ernest Rutherford and others worked on

figuring out what radioactivity was. • Discovered that radioactive emissions were

produced from unstable nuclei. • Several types of radioactivity alpha (α) decay beta (β) decay gamma (γ) ray emission positron emission electron capture

II. Review of Atomic Symbols

II. Subatomic Particles

• The term nuclide is used to refer to a particular isotope of an element.

• Each nuclide is composed of subatomic particles.

• Each subatomic particle has its own representation in nuclear chemistry.

p 1 1

n 0 1 e -1

0

II. Shedding Helium

II. Nuclear Equations

• In a nuclear reaction, elements change their identity.

• Nuclear equations are balanced by ensuring the sum of mass numbers and the sum of atomic numbers on both sides are equal.

II. α Particles – Dangerous?

• Alpha particles are the most massive particles emitted by nuclei.

• They have the potential to interact with and damage other molecules.

• Alpha radiation has the highest ionizing power, but it has the lowest penetrating power.

II. Emitting an Electron

II. Dangers of Beta Particles

• Beta particles are less massive than alpha particles, so they have less ionizing power.

• However, they have greater penetrating power. Sheet of metal or thick block of wood needed to stop them.

II. Gamma Ray Emission • This type of radiation involves emission

of high-energy photons, not particles. • Gamma rays have no mass and no

charge as they are a type of EM radiation.

• Gamma rays can be emitted along with other types of radiation.

• Gamma rays have low ionizing power, but very high penetrating power.

II. Antiparticles of Electrons!!

II. Electron Capture

• Instead of emitting particles, a nucleus can pull in an e- from an inner orbital.

• When an e- combines with a proton in the nucleus, a neutron is formed. proton + electron neutron

II. Radioactive Decay Summary

II. Sample Problems 20.1

a) Write a nuclear equation for the positron emission of sodium-22.

b) Write a nuclear equation for electron capture in krypton-76.

c) Potassium-40 decays into argon-40. Identify the type of radioactive decay.

III. Why Is There Radioactivity?

• When a nuclide undergoes radioactive decay, it becomes more stable.

• The strong force binds protons and neutrons together, but it only works at very short distances.

• Stability of nucleus is a balance between +/+ repulsions and the strong force attraction.

III. Importance of Neutrons

• Neutrons are key to nuclei stability because they increase strong force attractions, but lack charge repulsion.

• However, since neutrons occupy energy levels like e-, cannot just stuff nucleus with neutrons.

• Nuclear stability is indicated by the ratio of neutrons to protons (N/Z).

III. The Valley of Stability

• For lighter elements, N/Z for stable isotopes is about 1.

• For Z > 20, stability requires higher N/Z.

• No stable isotopes above Z = 83.

• Thus, nuclides decay to get back to the valley of stability.

III. Sample Problem 20.2

• If a nuclide has an N/Z ratio that is too high, what nuclear process is most likely to occur?

• If a nuclide has an N/Z ratio that is too low, what nuclear process is most likely to occur?

III. Magic Numbers • Nucleons occupy energy levels in the nucleus,

so certain numbers of nucleons are stable. • N or Z = 2, 8, 20, 28, 50, 82, and N = 126 are

uniquely stable and are called magic numbers.

III. Journey to Valley of Stability • Atoms w/ Z > 83

undergo decay in one or more steps to become stable.

• The successive decays to become stable are known as a decay series.

• Some steps involve gamma decay to remove extra energy.

IV. Detecting Radioactivity

Film-badge dosimeter

Geiger-Müller counter

IV. Radioactivity is Everywhere

• Everything around us contains at least some nuclides which are radioactive.

• Radioactivity is found in the ground, in our food, in our air.

• Radioactivity is in our environment because of some long decay times, and the constant production of radioactive nuclides through various decay series.

IV. Radioactivity is 1st Order

• All radioactive nuclides follow 1st order kinetics.

• Thus, ln Nt/N0 = -kt. • Since decay is 1st

order, half lives are independent of initial concentration.

IV. Sample Problem 20.3

• How long would it take for a 1.35-mg sample of Pu-236 to decay to 0.100 mg if it has a half-life of 2.87 years?

IV. Rate of Decay and Amount are Interchangeable

IV. Radiocarbon Dating • Radioactive C-14 is continuously taken

up by living organisms, so the amount is in equilibrium with the amount in the atmosphere (created by neutron bombardment of N-14).

• When the organism dies, it no longer takes in C-14. The C-14 continuously decays in the remains.

• Age can be determined by comparing rate of decay in remains to rate of decay in atmosphere.

IV. Sample Problem 20.4

• An ancient scroll is claimed to have originated from Greek scholars in about 500 B.C. A measure of its C-14 decay rate gives a value that is 89% of that found in living organisms. How old is the scroll? Could it be authentic? Note that the half-life for C-14 is 5730 years.

IV. Uranium/Lead Dating

• C-14 dating is only good for things that are less than 50,000 years old.

• Can use other known radioactive decays to date older things.

• U-238 decays into Pb-206 with a half-life of 4.5 × 109 years.

IV. Sample Problem 20.5

• A rock contains a Pb-206 to U-238 mass ratio of 0.145:1.00. Assuming that the rock did not contain any Pb-206 at the time of its formation, determine its age. Note that the half-life of U-238 is 4.5 × 109 years.

V. Making New Elements • Enrico Fermi attempted to synthesize a

new element by bombarding U-238 with neutrons.

• He detected beta particles, but never confirmed the chemical products.

V. Nuclear Fission

• Meitner, Strassmann, and Hahn repeated Fermi’s experiment.

• They discovered that elements lighter than uranium were produced w/ a lot of energy.

V. Nuclear Chain Reaction

V. Source of Energy in Fission

• U-235 + n Ba-140 + Kr-93 + 3n • If we look at exact masses, we find that

mass of products is 235.86769 amu and mass of reactants is 236.05258 amu.

• Mass is not conserved!! • In nuclear reactions, mass can be

converted into energy via E = mc2.

V. The Mass Defect • All stable nuclei have masses less than

their components which is known as the mass defect.

• When the mass defect is used in E = mc2, the nuclear binding energy is calculated.

• Mass is converted to energy to hold nucleus together!

• The nuclear binding energy is the energy needed to break up a nucleus into its component nucleons.

V. Calculating Binding Energies

• A useful conversion between mass and energy is 1 amu = 931.5 MeV. Note that 1 MeV = 1.602 x 10-13 J.

• The mass defect of a helium nucleus is 0.03038 amu, so its binding energy is 28.30 MeV.

V. Comparing Nuclei Stability

• In order to see which nuclei are more stable than others, the binding energy per nucleon is calculated.

• This is simply the binding energy divided by the number of nucleons in the nuclide.

VI. Nuclear Fusion

• Smaller nuclides can combine into more stable nuclides in a process called fusion.

• Fusion is the energy source of the sun and used in hydrogen bombs.

• High temps are needed to overcome the +/+ repulsions.

VI. Tokamak Fusion Reactor

VII. Making New Elements

• Why did the alchemists fail at turning lead into gold?*

• Changing one element into another is known as transmutation.

• Early work in transmutation involved bombarding nuclides w/ alpha particles.

• Al-27 + alpha particle P-30 + neutron

VII. Linear Accelerators

VII. Sample Problem 20.6

• Write a balanced nuclear reaction for the creation of element 107 and one neutron from the collision of bismuth-209 with chromium-54.

VIII. Radiation Risks • There are 3 categories of radiation

effects. Acute radiation damage: large amounts of

radiation in short time. Immune and intestinal cells most susceptible. Increased cancer risk: low dose over time.

Damage occurs to DNA. Genetic defects: high radiation exposure to

reproductive cells causing problems in offspring. Not clearly seen in humans, even Hiroshima survivors.

VIII. Measuring Exposure • There are several ways to measure

exposure to radiation. curie (Ci): exposure to 3.7 x 1010 decay

events per second. gray (Gy): exposure to 1 J/kg body tissue.

Also have the rad (radiation absorbed dose) which is 0.01 J/kg body tissue. rem (roentgen equiv. man): multiplication

of rads by the biological effectiveness factor, which depends on the type of radiation.

VIII. Sources of Radiation

VIII. Results of Radiation Exposure

VIII. Applications of Radioactivity

• Medicine Use of radiotracers to track movement of

compound or mixture in body. I-131 for thyroid, labeled antibodies to locate infection, P-32 for cancer. Gamma rays to kill cancer cells.

• Kill microorganisms Sterilize medical devices. Kill bacteria and parasites in food.

• Sterilize harmful insects