NUCLEAR CHEMISTRY

Henri Becquerel Discovers natural radioactivity FYI: Historical Perspective

Marie Sklodowska, Polish chemist marries Pierre Curie, French physicist Marie died from leukemia caused by her exposure to radiation Pierre was killed while crossing the street when he was hit by a vegetable wagon.

FYI: Historical Perspective Ernest Rutherford 1899 – Discovers alpha, beta and gamma radiation

Nuclear Reactions Involve changes in the composition of nuclei Accompanied by the release of tremendous amounts of energy Two types: – Nuclear fission – Nuclear fusion

Nuclear Fission The splitting of a heavy nucleus into lighter nuclei

Nuclear Fusion The combination of light nuclei to produce a heavier nucleus

Nuclides Different atomic forms of all elements Most small nuclides have equal # of protons and neutrons Some nuclides have “magic #’s” of protons and neutrons and are especially stable

The neutron-to-proton ratio determines the stability of the nucleus For low atomic #’s: – Equal #’s of protons and neutrons Above atomic #20 (Calcium): – More neutrons than protons

Nuclei whose neutron-to-proton ratio is unstable undergo radioactive decay by emitting 1 or more particles and/or electromagnetic rays:

Type/ symbol IdentityMass (amu) ChargePenetration Alpha Beta Gamma Proton Neutron helium nucleus electron high energy radiation proton, H nucleus neutron low low-med high low-med very high

Comparing penetrating ability (low- high) and SHIELDING… or thin or block of lead piece of metal

Geiger counter Device used to measure the total amount of radiation a person has received

Nuclear Equations for Radioactive Decay

Alpha Particle Decay Example 1: Radium-226 transmutates by alpha decay. Write the nuclear equation that represents this process. or *the math must equal on both sides *the atomic number will change because of changing proton #, so it is a new element and symbol

Beta Particle Decay Example 2: Write the nuclear equation for the beta-decay of boron-12. or *the math must equal on both sides In β− decay, the weak interaction converts a neutron into a proton while emitting an electron

Gamma Radiation Example 3: Write the nuclear equation representing gamma radiation given off by the unstable radionuclide cobalt-60.

Nuclear Fission & Fusion

FISSION: a heavy nucleus splits into 2 lighter nuclei some elements undergo fission spontaneously some elements can be induced to undergo fission when bombarded with other particles (e.g. neutrons)

Nuclear Fusion (think stars) "Every time you look up at the sky, every one of those points of light is a reminder that fusion power is extractable from hydrogen and other light elements, and it is an everyday reality throughout the Milky Way Galaxy." Carl Sagan, Spitzer Lecture, October 1991

FUSION: 2 nuclei combine to form a heavier nucleus the sun is a tremendous fusion reaction; the major fusion reaction in the sun is thought to be: both fission & fusion release large amounts of energy (fusion more than fission)

The Atomic Bomb (FISSION) when the nucleus of U-235 splits, 2 isotopes are formed, plus neutrons are emitted these neutrons collide with other U-235 atoms, causing them to undergo fission; they release neutrons, and so on… The result - CHAIN REACTION!! The CRITICAL MASS is the minimum mass of fissionable material that must be used to sustain a chain reaction

The Atomic Bomb (FISSION)

Little Boy was the first nuclear weapon used in warfare. It exploded approximately 1,800 feet over Hiroshima, Japan, on the morning of August 6, 1945, with a force equal to 13,000 tons of TNT. Immediate deaths were between 70,000 to 130,000. Fat Man was the second nuclear weapon used in warfare. Dropped on Nagasaki, Japan, on August 9, 1945, Fat Man devastated more than two square miles of the city and caused approximately 45,000 immediate deaths. While Little Boy was a uranium (U-235) gun-type device, Fat Man was a more complicated and powerful plutonium (Pu-239) implosion weapon that exploded with a force equal to 20 kilotons of TNT. Atomic Bomb

Nuclear Power Plants (use nuclear reactions to generate electricity) *nuclear reactor *San Onofre, Ca Nuclear Generating Station. Situated near a body of water to use the water…

TO GENERATE ELECTRICITY: The energy released in the fission process generates heat, some of which can be converted into usable energy. This heat is used to boil water to produce pressurized steam. Steam turns turbine of a generator. Generator makes electricity.

Nuclear Power Plants contain nuclear reactors

Nuclear Reactors (FISSION) use subcritical masses of fissionable material (below the mass to create chain reactions) CORE of reactor: contains fuel pins made of U-235; interspersed among the pins are control rods control rods: absorb neutrons (regulates amount of energy produced) – pull rods out of core: fission increases (more energy in core) – push rods back into the core: fission decreases (less energy in core)

Safety feature: if power is lost, rods will automatically fall into the core and shut the reaction down. Water pumps help to prevent the overheating of the reactor core (can trigger an explosion: Chernobyl)

PROS OF NUCLEAR ENERGY: no air pollution enormous amt. of energy released alternative to using our rapidly decreasing fossil fuels

CONS OF NUCLEAR ENERGY containers for waste products may erode or break thermal pollution (heated water returned to rivers, etc.) potential theft of fuel (Pu-239) for use in weapons

Half-Life 25.2 Half-life (t 1/2 ): – The half-life of a sample of a radioactive isotope is the time required for half of it to decay – After each half-life, half of the existing radioactive atoms have decayed into atoms of a new element.

Use half-lives of some radioisotopes found in nature to determine the age of ancient artifacts The half-life of Uranium-238 = billions of years

Radioactive Decay of an Unstable Isotope See textbook pg. 804 Questions – 1. What percent of the atoms remains after 1 half- life?

50%

2. What percent of the atoms remains after 2 half-lives?

25%

3. Approximately how many half-lives does it take for 12.5% of the radioisotope to remain?

Three half-lives

Question Compared to the energy liberated per gram during a typical chemical reaction, the amount of energy liberated per gram during the decay of a radioisotope is: A. somewhat greater B. less C. much greater D. about the same

Answer C. much greater

Do #51 on pg. 822 of text Decay curve for thorium-234 A. what percent of the isotope remains after 60 days? B. How many grams of a 250 g sample of thorium- 234 would remain after 40 days had passed? C. How many days would pass while 44 g of thorium-234 decayed to 4.4 g of thorium-234 D. What is the half-life of thorium-234?

Answers A. about 20% B. about 85 g C. about 83 days D. about 25 days

Half-life CALCS Carbon-14 emits beta radiation and decays with a half-life of 5730 years. Assume you start with a mass of 2.00 x g of carbon-14 a. How long is 3 half-lives?

ANSWER A. 3 half-lives = 3 x 5730 years = 17,190 yrs

Carbon-14 emits beta radiation and decays with a half-life of 5730 years. Assume you start with a mass of 2.00 x g of carbon-14 b. How many grams of the isotope remain at the end of 3 half-lives?

ANSWER B. the initial mass of carbon-14 is reduced by one half for each of the 3 half-lives, so for 3 half-lives: Remaining mass = 2.00 x g x ½ x ½ x ½ = x g = 2.50 x g

Example #1 Manganese-56 is a beta emitter with a half- life of 2.6 hours. – What is the mass of manganese-56 in a 1.0 mg sample of the isotope at the end of 10.4 hours?

Answer 10.4 hours/(2.6 hours/half-life) = 4 half-lives 1.0 mg x (½ x ½ x ½ x ½) = mg Mn-56

Ex #2 A sample of thorium-234 has a half-life of 24.1 days. Will all the thorium undergo radioactive decay in 48.2 days? Explain

ANSWER 48.2 days/(24.1 days/half-life) = 2 half-lives NO!!! (½ x ½ ) or ¼ of the sample will remain.