Nuclear Chemistry What is nuclear chemistry Most chemical

Nuclear Chemistry

What is nuclear chemistry? Most chemical reactions involve exchange or sharing of electrons

What is nuclear chemistry? Nuclear chemistry is quite different because it involves changes in the nucleus.

The Three Mile Island nuclear plant on Thursday, March 11, 2004 in Pennsylvania's Londonderry Twp. It was 25 years ago on March 28, 1979 that an accident in the Unit Two nuclear reactor at the plant caused a near meltdown of the unit's core.

PRISCILLA was a 37 kiloton balloon shot fired June 24, 1957 at the Nevada Test Site.

Transmutation When the nucleus of one element is changed into the nucleus of a different element the reaction is called a transmutation.

Why do transmutations occur? Because of the stability of the nucleus

Most nuclei are stable The ratio of protons to neutrons determines the stability The ratio in all nuclei with atomic numbers greater than 83 makes them unstable

Radioactive Because of their instability, all nuclei with atomic numbers >83 are said to be radioactive.

Radioisotopes For any element, an isotope that is unstable (and thus radioactive) is called a radioisotope.

An unstable nucleus spontaneously decays, forming products that are more stable

When a nucleus decays, it emits radiation in the form of Alpha particles Beta particles Gamma Particles

Alpha Particle Is a helium nucleus composed of two protons and two neutrons It is represented by the 4 He, symbol 2 or the symbol , which is the greek letter alpha

Beta Particle Is an electron whose source is an atomic nucleus It is represented by the symbol , which is the Greek letter beta.

What’s a positron? A positron is identical to an electron, except that it has a positive charge

Gamma Rays Almost all nuclear decay also releases some energy in the form of gamma rays Gamma rays similar to X-Rays, but have more energy.

Demo Geiger Counter

Right now I'm having amnesia and deja vu at the same time. I think I've forgotten this before. Steven Wright

Challenge Question is unstable and emits an alpha particle. 226 88 Ra Is the radium atom still a radium atom? If not, what is it?

Nuclear Equations Nuclear reactions can be represented by equations As in chemical equations, mass and charge must balance on both sides

Alpha Decay Example on overhead Alpha decay summary atomic number decreases by two number of protons decreases by two number of neutrons decreases by two mass number decreases by four

Beta Decay Summary atomic number increases by 1 number of protons increases by 1 number of neutrons decreases by 1 mass number remains the same

Sample Problem Sample of Beta Decay Sample of Positron Emmission

Positron Emission Summary atomic number decreases by 1 number of protons decreases by 1 number of neutrons increases by 1 mass number remains the same

Half-Life

Radioactive substances decay at a constant rate The rate of decay does not depend on temperature, pressure, or concentration

Decay of an individual nucleus is a random event – we can’t predict when its going to occur

However, the number of unstable nuclei that will decay in a given time in a sample of the element can be predicted.

Half-Life The time it takes for half of the atoms in a given sample to decay is called the half-life of the element

Half-Life Each isotope has its own half- life (See Reference Tables Table N) The shorter the half-life of an isotope, the less stable it is.

Chemistry Lab Half-Life of Red Licorice

Half-Life Equation The fraction remaining after a given number of half-lives is calculated using the relationship fraction remaining = (1/2)t/T where: t = total elapsed time T = half-life

Half-Life Equation (Con’t) The quantity t/T is the number of half-life periods = t/T (Reference Tables Table T)

Sample Problem Most chromium atoms are stable, but Cr-51 is an unstable isotope with a half-life of 28 days. ØWhat fraction of a sample of Cr-51 will remain after 168 days? ØIf a sample of Cr-51 has an original mass of 52. 0 g, what mass will remain after 168 days?

Sample Problem How much was present originally in a sample of Cr-51 if 0. 75 mg remains after 168 days?

Determining Half-Life Graphically Illustrate on overhead Reference Licorice Lab

Nuclear Energy

Challenge Question Why do nuclear reactions produce so much energy?

Artificial Radioactivity Elements can be made radioactive by bombarding their nuclei with high energy particles.

Artificial Transmutation In artificial transmutation, the nucleus is first bombarded with high energy particles, then decays and changes into a new element.

What “High energy particles”? Protons 1 Neutrons 1 n 0 1 H Alpha particles 2 4 He (Reference Tables Table O)

Example - Artificial Transmutation 13 27 Al + 24 He -------> 1530 P + 01 n

Sample Problem In the reaction 4 9 Be + X ----> 612 C + 01 n What does the X represent? Is this natural or artificial transmutation?

Nuclear Reactions and Energy Turns out (in the unusual case of a nuclear reaction) mass can be converted to energy E = mc 2

Nuclear Reactions and Energy Converting a little mass makes a lot of energy

Nuclear Reactions and Energy The total mass of a nucleus is less than the sum of the masses of the individual protons and neutrons. The matter that has been converted into energy is known as the mass defect.

Fission Reactions In a fission reaction, a high energy particle (usually a neutron) is captured by a nucleus, causing it to go unstable, and split. When the nucleus splits, a lot of energy is released.

Chemistry Lab Chain Reactions

Challenge Question Why are neutrons usually used to initiate a fission reaction? Answer: They’re neutral, and therefore aren’t repelled by the positive charge of the nucleus.

Fusion Reactions Involve the combining of light nuclei to form heavier ones. Best example - on the sun, hydrogen nuclei react in a series to form helium and release lots of energy.

Fusion Reactions Require extremely high temperatures and pressures Major advantage - products are not highly radioactive, like the products of fission reactions.

Activity With a partner, research the pro’s and cons of nuclear energy. In your opinion, should we use nuclear energy?

Chain Reaction Lab Page 805 in your textbook. Write out lab on separate sheet of paper Be sure to include your name, title of the lab, procedure (out of your text), summary of your findings, and your answers to the analysis questions

Uses and Dangers of Radioisotopes

Animals and plants have a known proportion of Carbon-14 (a radioisotope of Carbon) in their tissues. When they die they stop taking Carbon in, then the amount of Carbon-14 goes down at a known rate (Carbon-14 has a halflife of 5700 years). The age of the ancient organic materials can be found by measuring the amount of Carbon-14 that is left. Dating

Radioactive Tracers Radioisotopes can be used for medical purposes, such as checking for a blocked kidney. To do this a small amount of Iodine-123 is injected into the patient, after 5 minutes 2 Geiger counters are placed over the kidneys.

Radioactive Tracers The most common tracer is called Technetium-99 and is very safe because it only emits gamma rays and doesn't cause much ionization.

Radioactive Tracers Also radioisotopes are used in industry, to detect leaking pipes. To do this, a small amount is injected into the pipe. It is then detected with a GM counter above ground.

Smoke alarms contain a dustrial Applications weak source made of Americium-241. Alpha particles are emitted from here, which ionize the air, so that the air conducts electricity and a small current flows. If smoke enters the alarm, this absorbs the alpha particles, the current reduces, and the alarm sounds. Am-241 has a half-life of 460 years.

Industrial Applications In paper mills, the thickness of the paper can be controlled by measuring how much beta radiation passes through the paper to a Geiger counter.

Industrial Applications Gamma rays are also used to sterilize hospital equipment, especially plastic syringes that would be damaged if heated.

Industrial Applications Even after it has been packaged, gamma rays can be used to kill bacteria, mould and insects in food. This process prolongs the shelf-life of the food, but sometimes changes the taste.

Industrial Applications Checking Welds If a gamma source is placed on one side of the welded metal, and a photographic film on the other side, weak points or air bubbles will show up on the film, like an Xray.

Medical Applications Because Gamma rays can kill living cells, they are used to kill cancer cells without having to resort to difficult surgery. This is called "Radiotherapy", and works because cancer cells can't repair themselves when damaged by gamma rays, as healthy cells can.

Medical Applications I-131 is used in both detection and treatment of thyroid conditions, because I-131 accumulates in the thyroid gland. Cobalt-60 emits large amounts of gamma radiation that can be aimed at cancerous tumors.

Dangers of Radioisotopes

Alpha Particles are slow, have a short range in air, and can be stopped by a sheet of paper.

Alpha Particles However, they can ionize the atoms in the cells in your body (such as a DNA molecule), scrambling the DNA’s instructions to the body.

Beta Particles Have a longer range than alpha’s, but ionize much less strongly. However, they do have more penetrating power, which means that they can get through your skin and affect cells inside you.

Gamma Rays Hardly ionize atoms at all However, they are very difficult to stop (requires thick lead and/or concrete).

Gamma Rays When they are absorbed by an atom, that atom gains quite a bit of energy, and may then emit other particles. If that atom is in one of your cells, this is not good!