The isotopes of an element are all the atoms that have in their nucleus the number of protons atomic number corresponding to the chemical behavior of that element.
However, the isotopes of a single element vary in the number of neutrons in their nuclei. Since they still have the same number of protons, all these isotopes of an element have identical chemical behavior. But since they have different numbers of neutrons, these isotopes of the same element may have different radioactivity.
An isotope that is radioactive is called a radioisotope or radionuclide. Two examples may help clarify this. The most stable isotope of uranium, U, has an atomic number of 92 protons and an atomic weight of 92 protons plus neutrons.
The isotope of uranium of greatest importance in atomic bombs, U, though, has three fewer neutrons. Thus, it also has an atomic number of 92 since the number of protons has not changed but an atomic weight of 92 protons plus only neutrons.
The chemical behavior of U is identical to all other forms of uranium, but its nucleus is less stable, giving it higher radioactivity and greater susceptibility to the chain reactions that power both atomic bombs and nuclear fission reactors. Another example is iodine, an element essential for health; insufficient iodine in one's diet can lead to a goiter.
Iodine also is one of the earliest elements whose radioisotopes were used in what is now called nuclear medicine. The most common, stable form of iodine has an atomic number of 53 protons and an atomic weight of 53 protons plus 74 neutrons. Because its nucleus has the "correct" number of neutrons, it is stable and is not radioactive.
A less stable form of iodine also has 53 protons this is what makes it behave chemically as iodine but four extra neutrons, for a total atomic weight of 53 protons and 78 neutrons. With "too many" neutrons in its nucleus, it is unstable and radioactive, with a half-life of eight days. Because it behaves chemically as iodine, it travels throughout the body and localizes in the thyroid gland just like the stable form of iodine.
But, because it is radioactive, its presence can be detected. Iodine thus became one of the earliest radioactive tracers. PET scans are frequently combined with CT scans, with the PET scan providing functional information where the radioisotope has accumulated and the CT scan refining the location. The primary advantage of PET imaging is that it can provide the examining physician with quantified data about the radiopharmaceutical distribution in the absorbing tissue or organ. Radioisotopes Different isotopes of the same element have the same number of protons in their atomic nuclei but differing numbers of neutrons.
How do radioisotopes occur? Radioactive decay Atoms with an unstable nucleus regain stability by shedding excess particles and energy in the form of radiation. How are radioisotopes used? Radioisotope Half-life Use Hydrogen-3 tritium Carbon 5, years Used to measure the age of organic material up to 50, years old. Chlorine , years Used to measure sources of chloride and the age of water up to 2 million years old.
Lead Chromium Manganese Produced in reactors. Cobalt 5. Also used to irradiate fruit fly larvae in order to contain and eradicate outbreaks, as an alternative to the use of toxic pesticides.
Zinc Produced in cyclotrons. Technetiumm 6. Produced in 'generators' from the decay of molybdenum, which is in turn produced in reactors. Caesium Ytterbium Iridium Also used to trace sand to study coastal erosion. Gold 2. Also used to trace factory waste causing ocean pollution, and to study sewage and liquid waste movements.
Americium Radioisotope Half-life Use Phosphorus Yttrium 64 hours Used for liver cancer therapy. Molybdenum Iodine 8. Samarium Lutetium 6. Used to treat a variety of cancers, including neuroendocrine tumours and prostate cancer.
Radioisotope Half-life Use Carbon Also used to detect heart problems and diagnose certain types of cancer. Nitrogen 9. Oxygen 2. Fluorine 1. Used in a variety of research and diagnostic applications, including the labelling of glucose as fluorodeoxyglucose to detect brain tumours via increased glucose metabolism. As it turns out, the nature of isotopes — their chemical uniformity, their nuclear distinctiveness — makes them useful for a wide range of applications in fields as diverse as medicine, archaeology, agriculture, power generation and mining.
If you have ever had a PET scan , you have benefited from a byproduct of the radioactive decay of certain isotopes often called medical isotopes. We produce these medical isotopes using our knowledge of how nuclear reactions proceed, with the help of nuclear reactors or accelerators called cyclotrons.
But we have also found ways to make use of naturally occurring radioactive isotopes. Carbon dating , for example, makes use of the long-lived isotope carbon to determine how old objects are. Under normal circumstances, carbon is produced in our atmosphere via cosmic ray reactions with nitrogen It has a half-life of roughly 5, years, which means that half of a quantity of carbon will have decayed away in that time period.
While a biological organism is alive, it takes in approximately one carbon isotope for every trillion stable carbon isotopes and the carbon to carbon ratio stays about the same while the organism lives. Once it dies, new intake of carbon stops.
This means the ratio of carbon to carbon changes in the remains of this organism over time. If we extract carbon using chemical methods from a sample, we can then apply a method called accelerator mass spectrometry AMS to separate out the individual carbon isotopes by weight. AMS makes use of the fact that accelerated particles with the same charge but different masses follow separate paths through magnetic fields. By making use of these separate paths, we can determine isotope ratios with incredible accuracy.
As you can see from these examples, we apply our knowledge of isotopes in a variety of ways. We produce them, detect them, extract them, and study them with the dual purpose of understanding why the atomic nucleus behaves as it does, and how we can harness its power for our benefit.
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