SAT Chemistry Atomic Structure and the Periodic Table of the Elements - Nuclear Transformations and Stability
SAT Chemistry Atomic Structure and the Periodic Table of the Elements - Nuclear Transformations and StabilityAt the same time advances in atomic theory were occurring, scientists were noticing phenomena associated with emissions from the nucleus of atoms in the form of “X-rays.” While Roentgen announced the discovery of X-rays, Becquerel was exploring the phosphorescence of some materials. Becquerel’s work received little attention until early in 1898, when Marie and Pierre Curie entered the picture.
Searching for the source of the intense radiation in uranium ore, Marie and Pierre Curie used tons of it to isolate very small quantities of two new elements, radium and polonium, both radioactive. Along with Becquerel, the Curies shared the Nobel Prize in physics in 1903.
THE NATURE OF RADIOACTIVE EMISSIONS
While the early separation experiments were in progress, an understanding was slowly being gained of the nature of the spontaneous emission from the various radioactive elements. Becquerel thought at first that there were simply X-rays, but THREE different kinds of radioactive emission, now called alpha particles, beta particles, and gamma rays, were soon found. We now know that alpha particles are positively charged particles of helium nuclei, beta particles are streams of high-speed electrons, and gamma rays are high-energy radiations similar to X-rays. The emission of these three types of radiation is depicted below.
The important characteristics of each type of radiation can be summarized as follows:
METHODS OF DETECTION OF ALPHA, BETA, AND GAMMA RAYS
All methods of detection of these radiations rely on their ability to ionize. Three methods are in common use.
- Photographic plate. The fogging of a photographic emulsion led to the discovery of radioactivity. If this emulsion is viewed under a high-power microscope, it is seen that beta and gamma rays cause the silver bromide grains to develop in a scattered fashion.
- Scintillation counter. A fluorescent screen (e.g., ZnS) will show the presence of electrons and X-rays, as already mentioned. If the screen is viewed with a magnifying eyepiece, small flashes of light, called scintillations, will be observed. By observing the scintillations, one not only can detect the presence of alpha particles, but also can actually count them.
- Geiger counter. This instrument is perhaps the most widely used at the present time for determining individual radiation. Any particle that will produce an ion gives rise to an avalanche of ions, so the type of particle cannot be identified. However, each individual particle can be detected.
DECAY SERIES, TRANSMUTATIONS, AND HALF-LIFE
The nuclei of uranium, radium, and other radioactive elements are continually disintegrating. It should be emphasized that spontaneous disintegration produces the gas known as radon. The time required for half of the atoms of a radioactive nuclide to decay is called its half life.
For example, for radium, we know that, on the average, half of all the radium nuclei present will have disintegrated to radon in 1,590 years. In another 1,590 years, half of this remainder will decay, and so on. When a radium atom disintegrates, it loses an alpha particle, which eventually, upon gaining two electrons, becomes a neutral helium atom. The remainder of the atom becomes radon.
Such a conversion of an element to a new element (because of a change in the number of protons) is called a transmutation. This transmutation can be produced artificially by bombarding the nuclei of a substance with various particles from a particle accelerator, such as the cyclotron.
The following uranium-radium disintegration series shows how a radioactive atom may change when it loses each kind of particle. Note that an atomic number is shown by a subscript (92U), and the isotopic mass by a superscript (238U). The alpha particle is represented by the Greek symbol a, and the beta particle by β.
A helpful application of radioactive decay is in the determination of the ages of substances such as rocks and relics that have bits of organic material trapped in them. Because carbon- 14 has a half-life of about 5,700 years and occurs in the remains of organic materials, it has been useful in dating these materials. A small percentage of CO2 in the atmosphere contains carbon-14. The stable isotope of carbon is carbon-12. Carbon-14 is a beta emitter and decays to form nitrogen-14:
In any living organism, the ratio of carbon-14 to carbon-12 is the same as in the atmo-sphere because of the constant interchange of materials between organism and surround¬ings. When an organism dies, this interaction stops, and the carbon-14 gradually decays to nitrogen. By comparing the relative amounts of carbon-14 and carbon-12 in the remains, the age of the organism can be established. Carbon-14 has a half-life of 5,700 years. If a sample of wood had originally contained 5 grams of carbon-14 and now had only half or 2.5 grams of carbon-14, its age would be 5,700 years. In other words, the old wood emits half as much beta radiation per gram of carbon as that emitted by living plant tissues. This method was used to determine the age of the Dead Sea Scrolls (about 1,900 years) and has been found to be in agreement with several other dating techniques.
Nuclear fission reactions have been in use since the 1940s. The first atomic bombs used in 1945 were nuclear fission bombs. Since that time, many countries, including our own, have put nuclear fission power plants into use to provide a new energy source for electrical energy. Basically, a nuclear fission reaction is the splitting of a heavy nucleus into two or more lighter nuclei.
U-235 is bombarded with slow neutrons to produce Ba-139, Kr-94, or other isotopes and also 3 fast-moving neutrons.
A nuclear chain reaction is a reaction in which an initial step, such as the reaction above, leads to a succession of repeating steps that continues indefinitely. Nuclear chain reactions are used in nuclear reactors and nuclear bombs.
A nuclear fusion reaction is the combination of very light nuclei to make a heavier nucleus. Extremely high temperatures and pressures are required in order to overcome the repulsive forces of the two nuclei. Fusion has been achieved only in hydrogen bombs. Scientists are still trying to harness this reaction for domestic uses. The following examples show basically how the reactions occur.
The energy? released in a nuclear reaction (either fission or fusion) comes from the fractional amount of mass converted into energy. Nuclear changes convert matter into energy. Energy released during nuclear reactions is much greater than the energy released during chemical reactions.