Carbon-12 mass scale

Dalton s hydrogen-based atomic weight scale was eventually replaced by a scale based on oxygen and then, in 1961, by the present carbon-12 mass scale. This scale depends on measurements of atomic mass by an instrument called a mass spectrometer (Figure 2.10), which we will describe briefly later in this section. You make accurate measurements of mass on this instrument by comparing the mass of an atom to the mass of a particular atom chosen as a standard. On the present... 

In fact, atomic masses are not integers. On the atomic scale, carbon is given a value of 12.0000. On this accurate mass scale, oxygen is 15.9949, nitrogen is 14.0031, hydrogen is 1.0078, and so on. 

To set up a scale of atomic masses, it is necessary to establish a standard value for one particular species. The modem atomic mass scale is based on the most common isotope of carbon, 1 C. This isotope is assigned a mass of exactly 12 atomic mass units (amu) ... 

This small quantity is not easy to work with so, as you saw in Chapter 3, a scale called the relative atomic mass scale is used. In this scale an atom of carbon is given a relative atomic mass, An of 12.00. All other atoms of the other elements are given a relative atomic mass compared to that of carbon. 

You have already seen in Chapter 3 how we can compare the masses of all the other atoms with the mass of carbon atoms. This is the basis of the relative atomic mass scale. Chemists have found by experiment that if you take the relative atomic mass of an element in grams, it always contains 6 x 1023 or one mole of its atoms. 

For miscible blend phases, these parameters need to be described as a function of the blend composition. In a first approach to describe the behavior of the present PPE/PS and SAN/PMMA phases, these phases will be regarded as ideal, homogeneously mixed blends. It appears reasonable to assume that the heat capacity, the molar mass of the repeat unit, as well as the weight content of carbon dioxide scale linearly with the weight content of the respective blend phase. Moreover, a constant value of the lattice coordination number for PPE/PS and for SAN/PMMA can be anticipated. Thus, the glass transition temperature of the gas-saturated PPE/SAN/SBM blend can be predicted as a function of the blend composition (Fig. 17). Obviously, both the compatibilization by SBM triblock terpolymers and the plasticizing effect of the absorbed carbon dioxide help to reduce the difference in glass transition temperature between PPE and SAN. 

It is now recommended by the International Union of Pure and Applied Chemistry that the nuclide should be used as the basis of a unified scale. To bring them to the carbon-12 scale, atomic masses on the chemical scale must be reduced by 43 parts per million (multiplied by 0.999957). No atomic mass is changed by more than 4 in the last place quoted in the 1957 table, and few of these have been established with certainty to better than 5 in the last place. The carbon-12 scale is very suitable for expressing the masses of nuclides and has the advantage that very few of the present figures are affected by as much as their limits of error. 

Carbon, which composes diamond, is the basis for the atomic mass scale that is used today. 

If we know the atomic weight of an element on the carbon-12 scale, we can use the mole concept and Avogadro s number to calculate the average mass of one atom of that element in grams (or any other mass unit we choose). 

Figure 5-9 Mass spectrum of neon (1+ ions only). Neon consists of three isotopes, of which neon-20 is by far the most abundant (90.48%). The mass of that isotope, to five decimal places, is 19.99244 amu on the carbon-12 scale. The number by each peak corresponds to the fraction of all Ne+ ions represented by the isotope with that mass.

Mass defect is defined as the difference between the exact molecular weight and nominal molecular weight of an element. The atomic mass scale defines carbon-12 with a mass of exactly 12.0000 Da, therefore all other elements will have a uniquely different mass defect. For example, the mass defects of hydrogen and oxygen are 0.007825 and —0.005085 Da, respectively. Therefore, oxidation will introduce a mass defect of—5.1 mDa, while glucuronidation will introduce a mass defect of -i- 32 mDa. The mass defects of common nonsynthetic (Phase 1) and synthetic (Phase II) metabolites typically fall within 50 mDa. Therefore with LC/MS instruments capable of high mass accuracy, it is possible to filter out matrix-related interference ions whose mass... 

This tiny mass is exactly equal to one-twelfth of the mass of one atom of carbon (isotope mass number 12). This means that the mass of one atom of gC is exactly 12 on the atomic mass scale. This is written as m( gC) = 12 u. 

The mass of one atom of an isotope on the atomic mass scale is called its Isotopic mass m. Some isotopic masses are listed in Table 3.2. Remember that the mass of atoms is defined with reference to a standard atom , i.e. one atom of carbon-12 for which m( gC) = 12.0000 u. 

The atomic weight of an element is the average weight of all the isotopic masses of the element, calculated on the basis of their relative abundance in nature. The atomic weights are set on a carbon-12 scale. This is the standard weight scale that is used worldwide to express atomic weights. Exploring this further, we can say that 12 atomic mass units (amu) make up the mass of one... 

Atomic mass units are based on something called the Carbon 12 scale, a worldwide standard that s been adopted for atomic weights. By international agreement, a carbon atom that contains 6 protons and 6 neutrons has an atomic weight of exactly 12 amu, so 1 amu is Yu of this carbon atom. I know, what do carbon atoms and the number 12 have to do with anything Just trust me. Because the mass in grams of protons and neutrons are almost exactly the same, both protons and neutrons are said to have a mass of 1 amu. Notice that the mass of an electron is much smaller than that of either a proton or neutron. It takes almost 2,000 electrons to equal the mass of a single proton. 

The relative atomic mass expresses masses of atoms as relative values using the carbon-12 atomic mass scale. Relative atomic masses (symbol A ) are simply pure numbers and do not have units. 

The relative atomic mass (symbol A ) is the weighted average mass of a sample of naturally occurring atoms on the carbon-12 scale. The relative atomic mass of an element is the weighted average of its isotopes compared to one-twelfth of the mass of one atom of carbon-12 ... 

In 1995, Japan has installed a production facility with an 80-million cell capacity for mass-scale production of practical rechargeable batteries using LiCoOj as the cathodes. These rechargeable batteries are rated at 3.6 V and have demonstrated a density exceeding 350 Wh/liter. Performance improvements have been observed with improvements in the carbon anode, electrolyte, and cathode materials. Battery scientists may deploy lithiated oxides of nickel and manganese on the basis of cost reductions and reduced environmental impact. 

AMU n The atomic mass unit (amu), a unit of mass equal to 1/12 the mass of the carbon atom of mass number 12. On the atomic mass scale C=12. 

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