ATOM

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Atoms are the smallest subdivision of matter that can still be recognized as a particular element. Atoms themselves can be broken down into smaller, subatomic particles. There are three subatomic particles of interest


1. Electrons 1- electric charge and .1 x 10-8 grams


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. Protons 1+ electric charge and 1.7x 10 �4 grams


. Neutrons carry no electric charge and weigh slightly more that a proton.


In the Bohr model, the atom is conceived as having most of its mass in an extremely small central region called the nucleus. The protons and neutrons are in the nucleus. The electrons are confined to orbits at prescribed distances from the nucleus. These orbits are known as levels or shells. Each shell has a letter designation The innermost shell called the K-shell may contain one or two electrons. The next shell is known as the L- shell; it is further from the nucleus and may contain up to eight electrons. The remaining shells for the electrons are M, N, O, and P. These shells can contain 18, , 50, and 7 electrons, respectively.


Each element is identified by the number of protons in the nucleus; and since all atoms are electrically neutral, each element must have the same number of electrons as protons. The number of protons in the nucleus is called the atomic number, symbol Z. Then umber of protons in the nucleus is called the atomic number, symbol A. Atoms characterized by their atomic number and their mass number are called nuclides. Nuclides with the same number of protons but differing numbers of neutrons are called isotopes. All isotopes of a particular element have almost identical chemical properties. Some nuclides are radioactive, that is, each atom eventually undergoes spontaneous disintegration.


Elements are known for all proton numbers (Z) from 1 to 105. Elements with proton numbers from 1 to exist in nature; thru 105 have been synthesized.


Isotopes of a particular element may be stable or may be radioactive. The following statements illustrate the distribution of stable and unstable isotopes


1. Elements of Z=1 to 8 have stable isotopes except element 4 (Technetium) and element 61 (Promethium)


. Elements with Z =84 to 105 have no stable isotopes.


. All elements have at least one radioactive isotopes


4. There are 105 elements; however, there are over 500 different isotopes. Some elements have only isotopes; some have over 0.


5. There are only about 50 stable isotopes, the rest are radioactive.


. RADIOACTIVITY The process of undergoing spontaneous transformation of the nucleus, generally with the emission of alpha or beta particles. Often accompanied by gamma rays.


WHY?


a. neutron and proton number


b. neutron to proton ratio


. SOURCES OF RADIONUCLIDES


A NATURAL


To exist in nature a radioactive nuclide must have a half-life comparable to the age to the earth or it must be continuously produced by some process. The age of the earth is estimated to be -4x 10 years. We can classify natural radionuclides into three groups


1. Decay chains


Most radioactive isotopes found in nature are members of one of three radioactive series. Each series consists of sequential transformations beginning with a long-lived parent and ending with a stable nuclides are formed. Some have very short half-lives. The decay ends when the stable end product forms.





. Long Half-lived Radio nuclides


Some radioactive isotopes exist because they are continuously produced. The typical production is by interaction with cosmic radiation. The most noteworthy is K-40.


. Continuously produced





Some radioactive isotopes exist because they are continuously produced. The typical production is by interaction with cosmic radiation. The most noteworthy is C-14.


B. MAN-MADE


Many of the radioactive isotopes are man-made. Typically they are produced either thru the fission reactor or thru irradiation in an accelerator.





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