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Understanding Radiation:  

Nuclides & Isotopes

Ionizing & Non-Ionizing Radiation

An atom that has an unbalanced ratio of neutrons to protons in the nucleus seeks to become more stable. The unbalanced or unstable atom tries to become more stable by changing the number of neutrons and/or protons in the nucleus. This can happen in several ways:

Whatever the mechanism, the atom is seeking a stable neutron to proton ratio. In changing the number of nucleons (protons and neutrons), the nucleus gives off energy in the form of ionizing radiation. The radiation can be in the form of alpha particles (2 protons and 2 neutrons), beta particles (either positive or negative), x-rays, or gamma rays.

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Is the atom still the same element?

Only sometimes. If there is a change in the number of protons, the atom becomes a different element with different chemical properties. If there is a change in the number of neutrons, the atom is the same element, but becomes a different isotope of that element. All isotopes of one element have the same number of protons but different numbers of neutrons. All isotopes of a certain element also have the same chemical properties but have varying radiological properties such as half-life, or type of radiation emitted.

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What if the protons and electrons of an atom are unbalanced?

Normally, the number of electrons and protons is the same, so the atom is balanced electrically. Sometimes electrons are added or removed, and the atom carries a negative or positive charge. These charged forms of an element are called 'ions' of the element. This change affects the way the atom reacts chemically, but does not affect the stability of the nucleus atom's radioactivity.

What are nuclides and radionuclides?

Nuclide is a term used to categorize different forms of atoms very specifically. Each nuclide has a unique set of characteristics

If any of these change, the atom becomes a different nuclide. Approximately 3,700 nuclides have been identified. Most of them are radionuclides, meaning they are unstable and undergo radioactive decay.

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How are isotopes and radioisotopes different?

Isotopes are sets of nuclides having the same number of protons, but different number of neutrons. In other words, the same atomic number but a different atomic mass. Each individual isotope is a separate nuclide. Isotopes that are unstable and undergo radioactive decay are called radioisotopes. A change in the number of neutrons does not affect the charge of the atom.

Every known element has isotopic forms (although some natural elements only have artificially-created isotopes), and heavier elements tend to have more isotopes than lighter elements. Naturally-occurring element have one isotope that is most common. In some cases, the dominant isotope accounts for all, or nearly all, of that elements found in nature. In other cases, the proportion may be nearly equal among two or more isotopes.

The atomic mass assigned to the element in the periodic table usually represents an average of the masses of its isotopes. The average has been adjusted (weighted) to reflect the relative abundance of the different isotopes in nature. Sometimes the mass of the most stable (longest-lived) isotope is listed. So, even though carbon-12 is the basis for the Atomic Mass Unit , the atomic mass of carbon is usually listed as 12.011, because of its isotopes.

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What are nuclear isomers?

Nuclear isomers are two nuclides that have different energy states, but have the same number of protons and the same number of neutrons. As a result, they undergo radioactive decay differently.

One of these nuclides is generally less stable and will decay very quickly, although both or neither may be unstable.

What are metastable isomers?

The nuclear isomer that decays very quickly (has a very short half-life) is sometimes referred to as being metastable, and marked with an "m" when written in the format described on the page Atomic Shorthand.

For example, the two forms of cobalt-58 are:

          27Co58, half-life = 71 days   

27Co58m, half-life = 9 hours

Sometimes the metastable isomer decays to the longer-lived (or stable) isomer. This type of decay is called 'isomeric transition. Because both isomers are identical (except for their energies), the existence of a metastable isomer may only be suspected because the energy it gives off is different from the energy given off by the more stable isomer.

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