2 ch 28 nuclear chemistry (def radioactivity)

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Nuclear ChemistryChapter 28

Comparison of Chemical and Nuclear ReactionsChemical Reactions Occur when bonds are broken or formed Involve only valence electrons Associated with small energy changes Nuclear Reactions Occur when nuclei combine, split, & emit radiation Can involve protons, neutrons, & electrons Associated with large energy changes

Atoms keeps same identity Atoms of one element are although they may gain, often converted into atoms lose, or share electrons, of another element and form new substances Temperature, pressure, concentration, and catalysts affect reaction rates Temperature, pressure, and catalysts do not normally affect reaction rates

Radioactivity Radioisotopes are isotopes that have an

unstable nucleus. They emit radiation to attain more stable atomic configurations in a process called radioactive decay. Radioactivity is the property by which an atomic nucleus gives off alpha, beta, or gamma radiation. Marie Curie named the process. In 1898, Marie & Pierre Curie identified 2 new

elements, polonium & radium.

The penetrating rays and particles emitted by a

radioactive source are called radiation.

Radioactivity (cont) The presence of too many or too few

neutrons, relative to the number of protons, leads to an unstable nucleus. The types of radiation are alpha (), beta (), or gamma (). An unstable nucleus loses energy by emitting radiation during the process of radioactive decay. Spontaneous and does not require any input

of energy.

The effect of an electric field on ,, and , radiation. The radioactive source in the shielded box emits radiation, which passes between two electrodes. Alpha radiation is deflected toward the negative electrode, radiation is strongly deflected toward the positive electrode, and radiation is undeflected.

Nuclear Equations For a nuclear reaction to be balanced,

the sum of all the atomic numbers and mass numbers on the right must equal the sum of those numbers on the left. To figure out the unknown isotope, you need to balance the equation.

Example

Natural Radioactive Decay Why The nucleus has many positively charged protons that are repelling each other. The forces that hold the nucleus together cant do its job and the nucleus breaks apart. All elements with 84 or more protons are unstable and will eventually undergo nuclear decay. How Alpha particle emission Beta particle emission Gamma radiation emission Positron emission (less common) Electron capture (less common)

Alpha radiation A type of radiation called alpha radiation

consists of helium nuclei that have been emitted from a radioactive source. These emitted particles, called alpha particles, contain 2 protons and 2 neutrons and have a double positive charge.

Alpha Radiation (cont) Because of their large mass and charge,

alpha particles do not tend to travel very far and are not very penetrating. They are easily stopped by a piece of paper

or the surface of skin. Radioisotopes that emit alpha particles are dangerous when ingested.

Alpha radiation occurs when an unstable nucleus emits a particle composed of 2 protons and 2 neutrons. The atom giving up the alpha particle has its atomic number reduced by two. Of course, this results in the atom becoming a different element. For example, Rn undergoes alpha decay to Po.

Beta Particles A beta particle is essentially an electron

thats emitted from the nucleus. A neutron is converted (decayed) into a proton & electronso the atomic number increases by 1 and the electron leaves the nucleus. Isotopes with a high neutron/proton ratio often undergo beta emission, because this decay allows the # of neutrons to be decreased by one & the # of protons to be increased by one, thus lowering the neutron/proton ratio.

Beta radiation occurs when an unstable nucleus emits an electron. As the emission occurs, a neutron turns into a proton.

Positron Emission A positron is essentially an electron that has a

positive charge instead of a negative charge. A positron is formed when a proton in the nucleus decays into a neutron & a positively charged electron. It is then emitted from the nucleus. The positron is a bit of antimatter (seen in Star Trek). When it comes in contact with an electron, both particles are destroyed with the release of energy.

Positron emission occurs when an unstable nucleus emits a positron. As the emission occurs, a proton turns into a neutron.

Positron emission tomography, also called PET imaging or a PET scan, is a diagnostic examination that involves the acquisition of physiologic images based on the detection of radiation from the emission of positrons. Positrons are tiny particles emitted from a radioactive substance administered to the patient.

Antimatter National Geographic Article When a particle and its antiparticle meet, they

annihilate each other and their entire mass is converted into pure energy. Compared to conventional chemical propulsion systems, antimatter energy would slash the travel time to Mars and back from roughly two years to a few weeks. The world's largest maker of antimatter, the Fermi National Accelerator Laboratory in Batavia, Illinois, makes only one billionth of a gram a year at a cost of $80 million.

This artist's concept of an antimatter-powered rocket ship looks like a big spaceborne linear accelerator. Credit: Laboratory for Energetic Particle Science at Pennsylvania State University.

An artist's concept of a robotic antimatter-powered probe sailing past planets in an imaginary nearby solar system. Credit: Laboratory for Energetic Particle Science at Pennsylvania State University.

Gamma Radiation Gamma radiation is similar to x-rays high

energy, short wavelength emissions (photons). The symbol is , the Greek letter gamma. It commonly accompanies alpha and beta emission, but its usually not shown in a balanced nuclear reaction. Some isotopes, such as Cobalt-60, give off large amounts of gamma radiation. Co-60 is used in the radiation treatment of

cancerthe gamma rays focus on the tumor, thus destroying it.

Gamma radiation occurs when an unstable nucleus emits electromagnetic radiation. The radiation has no mass, and so its emission does not change the element. However, gamma radiation often accompanies alpha and beta emission, which do change the element's identity.

Electron Capture Electron capture is a rare type of nuclear

decay in which an electron from the innermost energy level (1s) is captured by the nucleus. This electron combines with a proton to form a neutron. The atomic number decreases by one but the mass stays the same. Electrons drop down to fill the empty space in the 1s orbital, releasing energy.

Man-Made Radioactive Decay on Earth Fission Fusion Occurs naturally in space Powers the sun Supernovas allow atoms to fuse into

heavier elements, this is how the other elements came into existence

Fission Nuclear fission occurs when scientists

bombard a large isotope with a neutron. This collision causes the larger isotope to break apart into two or more elements. These reactions release a lot of energy. You can calculate the amount of energy produced during a nuclear reaction using an equation developed by Einstein: E=mc2

Einstein "The intuitive mind

is a sacred gift and the rational mind is a faithful servant. We have created a society that honors the servant and has forgotten the gift."

Chain Reactions A chain reaction is a continuing cascade

of nuclear fissions. This chain reaction depends on the release of more neutrons then were used during the nuclear reaction. Isotopes that produce an excess of neutrons in their fission support a chain reaction - fissionable. There are only two main fissionable isotopes used during nuclear reactions uranium-235 & plutonium-239.

Chain Reactions (cont) The minimum amount of fissionable

material needed to ensure that a chain reaction occurs is called the critical mass. Anything less than this amount is subcritical.

Chain Reaction Figure

Because of the tremendous amount of energy

Atomic Bombs

released in a fission chain reaction, the military implications of nuclear reactions were immediately realized. The first atomic bomb was dropped on Hiroshima,

Japan, on August 6, 1945.

In an atomic bomb, two pieces of a fissionable

isotope are kept apart. Each piece by itself is subcritical. When its time for the bomb to explode, conventional explosives force the two pieces together to cause a critical mass. The chain reaction is uncontrolled, releasing a tremendous amount of energy almost instantaneously.

Mushroom Cloud

Nuclear Power Plants If the neutrons can be controlled, then the

energy can be released in a controlled way. Nuclear power plants produce heat through controlled nuclear fission chain reactions. The fissionable isotope is contained in fuel rods in the reactor core. All the fuel rods together comprise the critical mass. Control rods, commonly made of boron and cadmium, are in the core, and they act like neutron sponges to control the rate of radioactive decay.

Nuclear Power Plants (cont) In the U.S., there are approximately 100 nuclear

reactors, producing a little more than 20% of the countrys electricity. Advantages No fossil fuels are burned.

No combustion products (CO2, SO2, etc) to pollute

the air and water.

Disadvantages Cost - expensive to build and operate. Limited supply of fissionable Uranium-235. Accidents (Three Mile Island & Chernobyl) Disposal of nuclear wastes

A nucl