Exact Nuclidic Masses

Order of appearance = CO, Na, C2H4 the order of increasing exact nuclidic masses. 

Before we see an example of this calculation, we need to note three points. One is that the masses of nuclides are normally reported in atomic mass units, u (formerly, amu). One atomic mass unit is defined as exactly the mass of one atom of carbon-12 ... 

Nuclidic mass = mass of a particular nuclide, relative to mass of 12C, exactly 12 u. Atomic masses, Ar, commonly... 

A mass spectrometer is an instrument that separates particles by mass and measures their individual relative masses. If a nuclidic mass of an unknown compound is known with great precision from mass spectrometry, the exact molecular formula can often be deduced directly from this information without resort to a quantitative chemical analysis. 

The masses of individual atoms are very small. Even the heaviest atom discovered has a mass less than 5 x 10-25 kg. Since 1 kg is 2.21b, the mass referred to is less than 1.10 x 10-24 lb. It is convenient to define a special unit in which the masses of the atoms are expressed without having to use exponents. This unit is called the atomic mass unit, referred to by the symbol u in the literature. It is defined as exactly the mass of a 12C atom. The mass of the 12C atom is taken to be exactly 12u the mass of the 23Na atom is 22.9898 u. Table 2-1 lists the masses of some nuclides to which reference will be made in this chapter, as well as others. 

The mass number A is equal to the number of nucleons, A = P + N, and is always an integer. The nuclide mass M, on the other hand, is the exact mass of the nuclides in universal atomic mass units u, and the atomic mass is the mean of the nuclide masses of the stable nuclides in their natural abundance. 

One atomic mass unit is defined as exactly the mass of one atom (nuclide) of C ... 

All atomic nuclei are made up of protons and neutrons (known collectively as nucleons) the only exception is the lightest hydrogen nucleus, which consists of a single proton. The atomic number (Z) of an atom is the number of protons present in its nucleus (also the number of electrons in the neutral atom). The sum of protons (Z) and neutrons N) in a nucleus is referred to as the mass number ( 4). The mass number should not be confused with the atomic or nuclidic mass, which is the mass of the atom relative to that of a atom (which is, by definition, exactly 12.000... atomic mass tmits, amu). 

NucUdic mass is the mass, in atomic mass units, u, of an individual atom relative to an arbitrarily assigned value of exactly 12 u for the nuclidic mass of carbon-12. 

The tabulated molar mass of an element divided by Avogadro s number is the average mass per atom of that element, but it is not the exact mass of an individual nucleus. There are two reasons for this. First, molar masses refer to neutral atoms. The tabulated molar mass of an element includes the mass of its electrons in addition to the mass of its nucleus. Consequently, the mass of Z electrons must be subtracted from the isotopic molar mass in computing the energy of formation of a nuclide. Second, molar masses of the elements are weighted averages of... 

As we have already seen, the isotopic mass also is the exact mass of an isotope. The isotopic mass is very close but not equal to the nominal mass of that isotope (Table 3.1). Accordingly, the calculated exact mass of a molecule or of a mono-isotopic ion equals its monoisotopic mass (Chap. 3.1.4). The isotope C represents the only exception from non-integer isotopic masses, because the unified atomic mass [u] is defined as of the mass of one atom of nuclide C. 

Fourier transform mass spectrometry (FTMS) is a rapidly growing technique of increasing analytical importance. Foremost among its many attributes are its high mass resolution and wide mass range capabilities, as well as its ability to store ions. This relatively new technique has been employed in a wide variety of applications, ranging from the exact mass measurement of stable nuclides to the determination of peptide sequences. The future holds considerable promise both for the expanded use of FTMS in a diverse range of chemical problems, as well as advances in the capabilities of the technique itself. 

Thus the mass of a 12C atom is exactly 12 u. The masses and abundances of some other nuclides are listed in Table 2-1. 

The relative atomic mass of a nuclide is close to (except for but not exactly equal to its mass number. 

The mass of an atom, in general, is not equal to the sum of the masses of its component protons, neutrons, and electrons. If we could imagine a reaction in which free protons, neutrons, and electrons combine to form an atom, we would find that for all nuclides except H the mass of the atom is slightly less than the mass of the component parts and also that a tremendous amount of energy is released when the reaction occurs. The loss in mass is exactly equivalent to the released energy, according to the Einstein equation,... 

The isotopic competitive method, whether the analyses are made mass spectrometrically or by radioactivity measurements, is very sensitive to impurities. In the use of radioactive isotopes, the sample to be counted must be decontaminated of other chemical species containing the same radioactive nuclide and it must be chemically pure to avoid dilution. Both of these effects must be reduced to the order of 0.1 per cent, which is a very exacting restriction. In the mass spectrometric method one must avoid impurities which give ion peaks (mjq), either from the parent or from a fragment, at the same mfe as the product analyzed. A scan of the mass spectrum usually enables one to detect such impurities and provides the basis for further purification processes. The latter must all be isotopically nonfractionating. 

Element Atomic weight Isotope (nuclide) Exact mass... 

An example of s-process reaction is given by Equation (2.38). The stable isotope Fe slowly increases its mass number by three consecutive neutron captures to form the unstable nuclide Fe. Because the neutrons are captured slowly, any unstable nucleus will have time to decay, which is exactly what happens in this case as Fe undergoes beta decay to form the stable Co nucleus. 

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