Nuclear Chemistry

Antoine Henri Becquerel (1852-1908)• February 1896. Antoine Henri Becquerel, a French scientist,

was conducting an experiment which started with the exposure of a uranium-bearing crystal to sunlight. Once the crystal had sat in the sunshine for a while, he placed it on a photographic plate. As he had anticipated, the crystal produced its image on the plate. Becquerel theorized that the absorbed energy of the sun was being released by the uranium in the form of x-rays.

• Further testing of this theory had to be put off for a few days because the sky had clouded up and the sun had disappeared. For the next couple of days he left his sample of uranium in a closed drawer along with the photographic plate.

• When the weather had cleared, he returned to the drawer to retrieve his gear. He was surprised to find that the crystal had left a clear, strong image on the photographic plate.

• How could this be? There was no source of energy to produce the image! What Becquerel had discovered was that a piece of mineral which contained uranium could produce it's image on a photographic plate in the absence of light. What he had discovered was radioactivity! He attributed this phenomenon to spontaneous emission by the uranium.

Pierre Curie (1859-1906)Marie Curie (1867-1934)• The husband and wife team of Pierre and Marie Curie became

interested in Becquereal's discovery. While experimenting with their own uranium-containing ore, they came up with the term "radioactivity" to describe the spontaneous emissions that they studied. This word is still used today to describe this special characteristic of some elements.(radioisotopes).

• While comparing the activity of pure uranium to a uranium ore sample, they found that the ore was significantly more radioactive than the pure material. They concluded that the ore contained additional radioactive components besides the uranium. This observation led to the discovery of two new radioactive elements which they named polonium and radium.

Did you know this??

• In 1903, Becquerel and the Curies together received the Nobel Prize in physics. This award was for their discovery of radioactivity and their other contributions in this area.

• Marie Curie received a second Nobel Prize in 1911 for the discovery of polonium and radium. She was the first person to win two Nobel Prizes.

• Did you know that the Curie's had a word named after them? The curie is a basic unit of measurement for describing radioactivity.

Types of Radiation

• Alpha particles: α • Positively charged …essentially the nucleus of a helium atom• 2 neutrons and 2 protons• They travel short distances, have large mass

• Beta particles: β-• Negatively charged….electrons emitted when a neutron decays to

form a proton and emits the beta particle

• Gamma radiation: γ• High energy radiation…more energy than x-rays!• Released by nucleus along with beta and alpha decay

Penetrating Power• Alpha radiation can be stopped by a sheet of paper; beta can

be stopped by a sheet of plastic; gamma is stopped by several inches of lead or several feet of concrete!

Natural Transmutation Transmutation = change of nucleus of an atom of an element

to another element (must involve a change in the number of protons!)

Natural transmutation = the change of naturally radioactive elements by emission of a radioactive particle

Isotopes of elements with atomic numbers less than 20 whose neutron: proton ratio is NOT 1:1 are likely to undergo spontaneous nuclear “decay” (i.e. loss of a radioactive particle) (for example…. carbon-12 is stable, carbon-14 is radioactive)

All isotopes of elements with atomic numbers > 82 are radioactive (“unstable”)

Beta Decay

•Converts a neutron to a proton and an electron

19 protons23 neutrons

20 protons22neutrons

Positron Decay

•Converts a proton to a neutron and a positron (anti-matter of electrons)

Balancing Nuclear Equations

• In order to balance nuclear transmutation reactions, the number of protons and total mass must remain the same on both sides of a nuclear equation.

Artificial Transmutation• Artificial transmutation = isotopes that are NOT naturally

radioactive can be transmutated by “bombardment” with nuclear particles.

Half-Life• amount of time required for one-half of the

isotope to undergo radioactive decay• The half-life of a radioisotope CANNOT be

changed!!• # of half-lives amount remaining

0 11 1/22 1/43 1/8

Half-Lives can vary….

How many half-lives will Sr-90 undergo in 112 years?

How many days must elapse for I-131 to undergo 3 half-lives?

Nuclear Energy• All nuclear energy is derived from the conversion

of small amounts of mass to energy.• Einstein’s relativity equation E = mc2 explains the relationship

between matter and energy (c = 3.00 x 108 m/s …it is the speed of light)

• When the mass of nuclear reactants (before) is compared to the products (after), a small amount of mass is unaccounted…this is called the mass defect and is the mass that is converted to energy

Nuclear Fission• All existing nuclear power

plants use nuclear fission as the source of energy

• Fission involves the splitting apart of large nuclei into smaller nuclei with the release of large amounts of energy

• A commonly used reaction involves the capture of a neutron by a U-235 nucleus and its splitting apart into Ba-141 and Kr-92 plus 3 neutrons

Fission

Pros

• Releases more energy than conventional fuels • No greenhouse gases• Viable source of energy

(we can do it!)

Cons

• U-235 accounts for only 3% of the world’s uranium • Non-renewable resource• Highly radioactive waste

products (difficult to dispose)

Nuclear Fusion• Fusion involves the joining

of small nuclei to release large amounts of energy

• One of the most common reactions involves the joining of H-2 (deuterium) and H-3 (tritium)atoms to form He-4 and a neutron

• Fusion powers our sun!!

Fusion pros

• Much more energy than fission• Abundant fuel (H-2 and

H-3) in sea water• Safe, non-toxic, stable

waste products

cons

• Requires extremely high pressures and temperatures to overcome repulsion by nuclei

Nuclear Accidents• March 1979• Three Mile Island, Pa• Partial meltdown

released radiation • No immediate deaths

• April 1986• Chernobyl, Ukraine• Worst nuclear

disaster in history, released more radiation, 31 deaths

Risks of Exposure to Radiation

How does radiation cause health effects?• alpha, beta and gamma radiation are forms of ionizing radiation,

which has enough energy to strip away electrons from atoms (creating ions) or to break some chemical bonds

• living tissue in the human body can be damaged by ionizing radiation …the body attempts to repair the damage, but sometimes the damage cannot be repaired or it is too severe or widespread

• mistakes made in the natural repair process can lead to cancerous cells

• because children are growing more rapidly (and their cells are dividing more rapidly), they are more sensitive to radiation effects

• radiation can also cause damage to DNA, leading to mutation in offspring

How much radiation is needed to cause health effects?

Exposure(rem) Health Effect Time to Onset

(without treatment)

5-10 changes in blood chemistry

50 nausea hours55 fatigue 70 vomiting 75 hair loss 2-3 weeks90 diarrhea 100 hemorrhage 400 possible death within 2 months

1,000 destruction of intestinal lining

internal bleeding and death 1-2 weeks

2,000 damage to central nervous system

loss of consciousness; minutes and death hours to days

Some radioactive elements are more dangerous than others• organs in the body use an element cannot distinguish between

radioactive and non-radioactive isotopes• radioactive iodine concentrates in the thyroid, because the

thyroid needs iodine to function normally • calcium, strontium-90 and radium-226 have similar chemical

properties. …so strontium and radium in the body tend to collect in calcium-rich areas, such as bones and teeth. They contribute to bone cancer

• radioisotopes with long half-lives emit radiation slowly, while those with short half-lives emit radiation more quickly

Calculate your exposure…• http://www.ans.org/pi/resources/dosechart/

Benefits of Radioisotopes

Medical Uses:• Iodine-131• because the thyroid uses iodine,

radioactive iodine can be used to detect and treat thyroid disorders

• Tc-99• most commonly used radioactive tracer• used to track blood flow in heart and brain• (tumor detection)

• Co-60• used as a source of gamma

radiation targeted to cancer cells.

• proper dosage can destroy cancer cells and preserve healthy cells

Benefits of Radioisotopes

• Tracers• used to follow pathways of chemical reactions

• Phosphorus-32• traces fertilizer uptake in plants

• Carbon-14• traces carbon in organic reactions

• Hydrogen-3 (tritium)• traces hydrogen in chemical reactions

Benefits of Radioisotopes• Dating• determines the age of objects

• Carbon-14:• percent of carbon 14 present in all

living things is constant• when an organism dies, carbon-14 is

no longer replenished• using ratio of carbon-14 to carbon 12,

the age of a fossil can be determined

• Uranium -238• used to determine the age of rocks

(“geologic formations”)

Miscellaneous Benefits• Americium-241• used in smoke detectors! • ionizes air, smoke disrupts the ionization, causing alarm to sound

• Radiation is used to kill bacteria in fresh fruits, vegetables and meats!