Carbon-12, elemental atomic weights based

Atomic Weights of the Elements Based on the Carbon 12 Standard... 

Note The calculation of relative molecular mass, Mr, of organic molecules exceeding 2000 u is significantly influenced by the basis it is performed on. Both the atomic weights of the constituent elements and the natural variations in isotopic abundance contribute to the differences between monoisotopic- and relative atomic mass-based values. In addition, they tend to characteristically differ between major classes of biomolecules. This is primarily because of molar carbon content, e.g., the difference between polypeptides and nucleic acids is about 4 u at Mr = 25,000 u. Considering terrestrial sources alone, variations in the isotopic abundance of carbon lead to differences of about 10-25 ppm in Mr which is significant with respect to mass measurement accuracy in the region up to several 10 u. [41]... 

A relative scale of atomic weights (as the weighted average of all forms, or isotopes, of a particular element found in nature) has been developed. The base of this scale is the assignment of a mass of 12.0000 to the isotope of carbon containing 5 protons, 5 neutrons, and 6 electrons. An atomic weight table can be found in Table 2.2. 

Thus, the mass observed for the molecular ion of CO, for example, is the sum of the exact formula masses of the most abundant isotope of carbon and of oxygen. This differs from a molecular weight of CO based on atomic weights that are the average of weights of all natural isotopes of an element (e.g., C = 12.01, O = 15.999). 

The periodic table provides atomic weights in atomic mass units. These atomic weights are average values, based on the contribution of all the isotopes of the particular element. For example, the average mass of a carbon atom is 12.01 amu and... 

The modern atomic weight system is based on the mass of the most common form of the element carbon. The mass of this form of carbon is defined to be exactly 12 atomic mass units, abbreviated as amu. On this scale, for example, hydrogen has an atomic weight of 1.0078 amu. The atomic masses of all the elements appear in Appendix B as the average masses relative to carbon twelve. We will examine the precise meaning of average mass in Sec. 2.4. 

Radiometric dating is a method that scientists use to measure the age of things. The most widely known form of radiometric dating is that based on carbon-14. It works like this. Carbon-12 is the most common form of carbon, and carbon-14 is what is called an isotope of it. (An isotope is a variation of the normal atom of an element, in that it has more or less neutrons than the standard atom.) Carbon-12 has six protons and six neutrons at its nucleus, and is therefore said to have an atomic weight of 12. The component of an atom that determines its character is the number of electrons in orbit around its nucleus. In carbon s case there are six. 

Molecular weight MW is the sum of the atomic weights of all the atoms in a molecule. It represents a measure of the chain length for the molecules that make up the polymer. Atomic weight is the relative mass of an atom of any element based on a scale in which a specific carbon atom is assigned a mass value of 12. 

Thomson used formulae for minerals based on Bergman s, in which A=alumina, S=silica, L=lime, etc. Thomson used quantitative formulae in his paper on oxalates . s =oxygen, c=carbon, A=hydrogen oxalic acid =4k +3C + 2A, sugar = 5 +3c+ 4/1, and used the initial letters of the names as symbols for atomic weights of the elements or acids and bases, also writing chemical formulae. Berzelius used the initial letter or letters of the Latinised names (at first Po, later K, for potassium, kalium). At first the figures were put... 

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