From the discussions in the previous section, we know that the atoms of any element have two distinct parts: the nucleus, which contains the protons and neutrons, and the extranuclear space, which contains the electrons.
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The electrons in the atom, particularly those farthest from the nucleus, determine the chemical properties of the element.
We will discuss electrons and the chemical properties of elements in detail in the next chapter.. In the remainder of this chapter, we will describe properties of the nucleus and, in particular, the characteristics of nuclear decay, which is also called radioactivity or radioactive decay of the nucleus.
In nuclear decay, the nuclei of radioactive atoms decay spontaneously to form other nuclei, a process that always results in a loss of energy and often involves the release of one or more small particles. Some atoms are naturally radioactive. Others that are normally stable can be made radioactive by bombarding them with subatomic particles. Often, one isotope of an element is radioactive and others of the same element are stable. A radioactive isotope is called a radioisotope.
Radioactivity is a common phenomenon. Of the 350 isotopes known to occur in nature, 67 are radioactive. Over a thousand radioactive isotopes have been produced in the laboratory. Every element, from atomic number 1 through number 109, has at least 1 natural or artificially produced radioactive isotope. Of the 3 known isotopes of hydrogen, one is radioactive - hydrogen-3, more commonly known as tritium.
Oxygen, the Earth's most abundant element, has 8 known isotopes, 5 of which are radioactive (oxygen-13, -14, -15, -19, and -20). Iodine, an element widely used in nuclear medicine, has 24 known isotopes ranging in mass from 117 to 139 amu. Of these, only iodine-127 is stable; this isotope is the only naturally occurring one. Uranium has 14 known isotopes, all of which are radioactive.