Isotopic abundances naturally occurring elements

ISOTOPES OF NATURALLY OCCURRING ELEMENTS AND THEIR ABUNDANCES... 

Isotopic ion. Any ion containing one or more of the less abundant naturally occurring isotopes of the elements that make up its structure. 

Uranium (symbol U atomic number 92) is the heaviest element to occur naturally on Earth. The most commonly occurring natural isotope of uranium, U-238, accounts for approximately 99.3 percent of the world s uranium. The isotope U-235, the second most abundant naturally occurring isotope, accounts for another 0.7 percent. A third isotope, U-234, also occurs uatiirally, but accounts for less than 0.01 percent of the total naturally occurring uranium. The isotope U-234 is actually a product of radioactive decay of U-238. 

The nominal mass is defined as the integer mass of the most abundant naturally occurring stable isotope of an element. [3] The nominal mass of an element is often equal to the integer mass of the lowest mass isotope of that element, e.g., for H, C, N, O, S, Si, P, F, Cl, Br, I (Table 3.1). The nominal mass of an ion is the sum of the nominal masses of the elements in its empirical formula. 

Natural Abundance. The isotopic abundances listed are on an atom percent basis for the stable nuclides present in naturally occurring elements in the earth s crust. 

The relative mass of one isotope of sulfur is 31.9721 u. Its abundance is 95.02%. Naturally occurring elemental sulfur has a relative atomic mass of 32.066. The mass number of the one other isotope of sulfur is... 

With atomic number 92 and atomic weight 238.03, uranium is the heaviest naturally occurring element. There are eleven known isotopes, of which three — with atomic weights 234, 235 and 238 — occur in nature. They are all radioactive, with half-lives (in years) of 2.35 X 10 , 7 X 10 and 4.5 X 10 , respectively. The relative abundance of the isotopes varies depending on the age and geological history of the uranium occurrence a typical distribution is U 99.28% U 0.71% U 0.005%. A quite exceptional deviation from this distribution has been found at Oklo in Gabon as a result of spontaneous fission chain reactions in the remote past (Anon., 1975). At Oklo U concentrations as low as 0.29% have been found. 

When you look up the atomic mass of carbon in a table such as the one on the inside front cover of this book, you will find that its value is not 12.(X) amu but 12.01 amu. The reason for the difference is that most naturally occurring elements (including carbon) have more than one isotope. This means that when we measure the atomic mass of an element, we must generally settle for the average mass of the naturally occurring mixture of isotopes. For example, the natural abundances of carbon-12 and carbon-13 are 98.90 percent and 1.10 percent, respectively. The atomic mass of carbon-13 has been determined to be 13.00335 amu. Thus the average atomic mass of carbon can be calculated as follows ... 

Since naturally occurring elements consist of mixtures of isotopes, the chemical atomic weights will be an average of the isotope weights of each element, taking into account their relative naturally occurring abundances. Thus, none of the... 

There are three expressions for mass that are important to know. The integer mass of the most abundant naturally occurring stable isotope is the nominal mass of the element. The exact mass of an isotope is determined by high-resolution MS the exact mass of Cl is 34.9689 Da and that for Cl is 36.9659 Da. The atomic weight is the... 

In all, iodine has 33 isotopes, of which 16 are radioactive. The one of interest is iodine-129 with a half-life of 15.7 x 10 years. Common iodine is iodine-127. Xenon is an inert gas present in the atmosphere in minute amounts of around 0.006 ppm. It has 36 isotopes, of which the one of interest is xenon-129 produced by iodine-129 via decay. This has an abundance in the naturally occurring element of 26.4 atom %. The I/Xe system could be a potentially precise geochronometer, especially of events in the solar system. 

Atoms are made up of numerous smaller particles of which the most important to chemical studies are the proton, neutron, and dectron. Positively chaiged protons and neutral neutrons have a relative mass of 1 u each and The atomic wdghts measured for elements are average weights that depend on the percentages and masses of the isotopes in the naturally occurring element. If the isotope percent abundances and isotope masses are known for an... 

The atomic weight can be calculated from the isotopic masses andfractional abundances of the isotopes in a naturally occurring element. These data can be determined by the use of a mass spectrometer, which separates ions according to their mass-to-charge ratios. 

Determining atonnc weight from isotopic masses and fractional abundances Given the isotopic masses (in atomic mass units) and fractional isotopic abundances for a naturally occurring element, calculate its atomic weight. (EXAMPLE 2.2)... 

Average Masses of Naturally Occurring Elements with Exact Masses and Representative Relative Abundances of Isotopes [1-3]... 

Mass and Relative Abundance of the Isotopes of All Naturally Occurring Elements... 

FIGURE 14.2 Relative isotopic abundances of the naturally occurring elements, showing all the potential isobaric interferences. (From UIPAC Isotopic Composition of the Elements, Pure and Applied Chemistry 75[6], 683-799, 2003.)... 

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