Protons, neutrons, and electrons are responsible for the chemical properties of atoms. They also have other properties that can be useful in a clinical setting. For example, they have been used to develop methods for examining the inside of the body.

Isotopes (i'so-topz) are two or more forms of the same element that have the same number of protons and electrons but a different number of neutrons. For example, there are three isotopes of hydrogen: hydrogen, deuterium, and tritium. All three isotopes have one proton and one electron, but hydrogen has no neutrons in its nucleus, deuterium has one neutron, and tritium has two neutrons. Isotopes can be denoted using the symbol of the element preceded by the mass number (number of protons and neutrons) of the isotope. Thus hydrogen is 1H, deuterium is 2H, and tritium is 3H.

Radioactive isotopes have unstable nuclei that spontaneously change to form more stable nuclei. As a result, either new isotopes or new elements are produced. In this process of nuclear change, three kinds of rays, called alpha, beta, and gamma rays, are emitted from the nuclei of radioactive isotopes. Alpha rays are a stream of positively charged helium ions (He²⁺), which consist of two protons and two neutrons. Beta rays are a stream of electrons formed as neutrons change into protons. The electrons are ejected from the nuclei, and protons remain in the nuclei. Gamma rays are a form of electromagnetic radiation released from nuclei as they lose energy.

All isotopes of an element have the same atomic number, and their chemical behavior is very similar. For example, tritium can substitute for hydrogen, and either 125 iodine or 131 iodine can substitute for 126 iodine in chemical reactions.

Radioactive isotopes are commonly used by clinicians and researchers because sensitive measuring devices can detect the radioactive rays emitted from isotopes, even when they are present in very small amounts. Several procedures that are used to determine the concentration of substances such as hormones depend on the incorporation of small amounts of radioactive isotopes such as hormones depend on the incorporation of small amounts of radioactive isotopes such as 125 iodine into the substances being measured. Disorders of the thyroid gland, the adrenal gland, and the reproductive organs can be more accurately diagnosed using these procedures.

Radioactive isotopes are also used to treat cancer. Some of the particles released from isotopes have a very high energy content and can penetrate and destroy tissues. Thus radioactive isotopes can be used to destroy rapidly growing tumors, which are more sensitive to radiation than healthy cells. Radiation can also be used to sterilize materials that cannot be exposed to high temperatures (for example, some fabric and plastic items used during surgical procedures). In addition, radioactive emissions provide a convenient and safe method of sterilizing food and other items.

X-rays are electromagnetic radiations with a much shorter wave-length than visible light. When electric current is used to heat a filament to very high temperatures, energy of the electrons becomes so great that some electrons are emitted from the hot filament. When these electrons strike a positive electrode, they release some of their energy in the form of x-rays.

X-rays do not penetrate dense material as readily as they penetrate less dense material, and x-rays can expose photographic fill. Consequently, an x-ray beam can pass through a person and onto photographic film. Dense tissues of the body absorb the x-rays, and in these areas the film is underexposed, appearing white or light in color on the developed film. On the other hand, the x-rays readily pass through less dense tissue, and the film in these areas is overexposed and appears black or dark in color. For example, in an x-ray of the skeletal system the dense bones are white, and the less dense soft tissues are dark, often so dark that no details can be seen. Because the dense bone material is clearly visible, x-rays can be used to determine if bones are broken or have other abnormalities.

Soft tissues can be photographed by using low-energy x-rays and radiopaque substances, which are dense materials that absorb x-rays. Mammograms are low-energy x-rays of the breast that can be used to detect tumors, because tumors are slightly more dense than normal tissue. If a radiopaque liquid is given to a patient, the liquid assumes the shape of the organ into which it is placed. For example, if a barium solution is swallowed, the outline of the upper digestive tract can be photographed using x-rays to detect abnormalities such as ulcers.

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