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Nuclear Chemistry Chapter 18. Larry Emme Chemeketa Community College. Discovery of Radioactivity.
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Nuclear Chemistry Chapter 18 Larry Emme Chemeketa Community College
In 1890, five years before Röentgen announced his discovery of the rays that made the field of radiology possible, a University of Pennsylvania physics professor and a photographer inadvertently exposed two coins to a photographic plate and produced an X-ray. Not understanding the accident, however, they filed the film, only to recall it and realize what they had done when Röentgen's work became public.
In 1895 Wilhelm Röentgen discovered X-rays. • Röentgen observed that a vacuum discharge tube enclosed in a thin, black cardboard box had caused a nearby piece of paper coated with the salt barium platinocyanide to phosphorescence.
From this and other experiments he concluded that certain rays, which he called X-rays, were emitted from the discharge tube, penetrated the box, and caused the salt to glow.
Known in Britain by the trade name ‘Pedoscope’. The machine produced an X-ray of the customer’s foot inside a shoe to ensure shoes fitted accurately, which both increased the wear-time of the shoe and with that, the reputation of the shoe shop. The customer placed their foot over an X-ray tube contained within the wooden base of the Pedoscope. From this, a beam of X-rays passed through the foot and cast an image onto a fluorescent screen above. The screen could be observed via three viewing points – one for the shoe-fitter, one for the customer, and one for a third party (usually the guardian of a child being fitted). The accommodation for three viewing points may seem a little extravagant, but it may be an indication of the popularity of the Pedoscope and the interest the public had in the machine.
Shoe-Fitting Fluoroscope (ca. 1930-1940) Basic Description The shoe fitting fluoroscope was a common fixture in shoe stores during the 1930s, 1940s and 1950s. A typical unit, like the Adrian machine shown here, consisted of a vertical wooden cabinet with an opening near the bottom into which the feet were placed. When you looked through one of the three viewing ports on the top of the cabinet (e.g., one for the child being fitted, one for the child's parent, and the third for the shoe salesman or saleswoman), you would see a fluorescent image of the bones of the feet and the outline of the shoes.
Shortly after Röentgen’s discovery, Antoine Henri Becquerel attempted to show a relationship between X-rays and the phosphorescence of uranium salts. • Becquerel wrapped a photographic plate in black paper, sprinkled a sample of a uranium salt on it, and exposed it to sunlight.
When Becquerel attempted to repeat the experiment the sunlight was intermittent. • He took the photographic plate wrapped in black paper with the uranium sample on it, and placed the whole setup in a drawer.
Several days later he developed the film and was amazed to find an intense image of the uranium salt on the plate. • He repeated the experiment in total darkness with the same result.
This proved that the uranium salt emitted rays that affected the photographic plate, and that these rays were not a result of phosphorescence due to exposure to sunlight. • Two years later, in 1896, Marie Curie coined the name radioactivity. Radioactivity is the spontaneous emission of particles and/or rays from the nucleus of an atom. Elements having this property are radioactive.
In 1899 Rutherford began to investigate the nature of the rays emitted by uranium. • He found two particles in the rays. He called them alpha and beta particles. • Rutherford’s nuclear atom description led scientists to attribute the phenomenon of radioactivity to reactions taking place in the nuclei of atoms.
The gamma ray, a third type of emission from radioactive material, was discovered by Paul Villard in 1900.
nucleon a proton or a neutron mass number the total number of nucleons in the nucleus.
nuclide any isotope of any atom isotope atoms of the same element with different masses
6 protons + 6 neutrons 12 C 6 6 protons A nuclide of carbon
8 protons + 8 neutrons 16 O 8 8 protons A nuclide of oxygen
8 protons + 9 neutrons 17 O 8 8 protons A nuclide of oxygen
8 protons + 10 neutrons 18 O 8 8 protons A nuclide of oxygen
In August 1932, Carl D. Anderson found evidence for an electron with a positive charge, later called the positron. Anderson discovered the positron while using a cloud chamber to investigate cosmic rays. According to this theory, a positron was a hole in a sea of ordinary electrons. The positron was the antimatter equivalent to the electron.
Radioactive elements continuously undergo radioactive decay or disintegration to form different elements. • Radioactivity is a property of an atom’s nucleus. It is not affected by temperature, pressure, chemical change or physical state.
radioactive decay the process by which a radioactive element emits particles or rays and is transformed into another element.
Each radioactive nuclide disintegrates at a specific and constant rate, which is expressed in units of half-life. • The half-life (t1/2) is the time required for one-half of a specific amount of a radioactive nuclide to disintegrate.
Willard Libby and his apparatus for carbon-14 dating (1946).
The amount of radioactive carbon-14 in the skeleton diminishes by ½ every 5730 years. The result is that the skeleton contains only a fraction of the carbon-14 it originally had. The red arrows symbolize the relative amounts of carbon-14.
Scientists are able to calculate the age of carbon-containing artifacts, such as wooden tools or skeletons, by measuring their current level of radioactivity. This process, carbon dating, enables one to probe as much as 50,000 years into the past. Beyond that time span, there is too little carbon-14 remaining to permit accurate analysis. The dating of older things is accomplished with radioactive minerals, such as uranium-238 and uranium-235 which decay very slowly.
Carbon-14 dating would be an extremely simple dating method if the amount of radioactive carbon in the atmosphere had been constant over the ages. The fact is, it has not. Fluctuations in the Sun’s magnetic field as well as changes in the strength of Earth’s magnetic field affect cosmic-ray intensities in Earth’s atmosphere, which in turn produce fluctuations of the in the production of C-14. Also, changes in Earth’s climate affect the amount of CO2 in the atmosphere. The oceans are enormous reservoirs of CO2. When the oceans are cold, they release less CO2 into the atmosphere than when they are warm.
Nuclides are said to be either stable (nonradioactive) or unstable (radioactive). • Elements that have an atomic number greater than 83 are naturally radioactive. • Some of the naturally occurring nuclides of elements 81, 82 and 83 are radioactive and some are stable.
Only a few naturally occurring elements that have atomic numbers less than 81 are radioactive. • No stable isotopes of element 43 (technetium) or of element 61 (promethium) are known.
Radioactivity is believed to be a result of an unstable ratio of neutrons to protons in the nucleus. • Stable nuclides of elements up to about atomic number 20 generally have a about a 1:1 neutron-to-proton ratio.
In elements above atomic number 20, the neutron-to-proton ratio in the stable nuclides gradually increases to about 1.5:1 in element number 83 (bismuth). • When the neutron to proton ratio is too high or too low, alpha, beta, or other particles are emitted to achieve a more stable nucleus.
three types of radiation are detected by a photographic plate Beta rays are strongly deflected to the positive pole. Marie Curie, in a classic experiment, proved that alpha and beta particles are oppositely charged. radiation passes between the poles of an electromagnet Gamma rays are not deflected by the magnet. Alpha rays are less strongly deflected to the negative pole. a radioactive source was placed inside a lead block
