Atomic mass definition

Use the fractional abundances and the atomic masses of fhe isotopes to compute the atomic mass according to the atomic mass definition given earlier. 

The atomic mass of an element is the weighted average mass of the isotopes of that element. Based on this definition, which of these does NOT show the correct atomic mass for an element ... 

How is the atomic mass unit defined How has this definition evolved ... 

Note Care has to be taken when mass values from dated literature are cited. Prior to 1961 physicists defined the atomic mass unit [amu] based on Vie of the mass of one atom of nuclide 0. The definition of chemists was based on the relative atomic mass of oxygen which is somewhat higher resulting from the nuclides and contained in natural oxygen. 

In introductory chemistry courses, much emphasis is necessarily placed on the concept of stoichiometry, that is, the fact that elements combine in certain definite proportions by weight, proportions that reflect their valences and atomic masses. For much of the chemistry of the main group elements and organic compounds, this concept works extremely well, but in transition metal chemistry in particular it is common for ions of more than one oxidation state to form with comparable ease, and sometimes to occur together in the same ionic solid. The presence of more highly oxidized cations... 

The atomic mass unit (mu) is also called the dalton (Da) - in honour of John Dalton. In response to the increase in the use of the name dalton for the unified atomic mass unit among chemists, it was suggested by IUPAC that the unified atomic mass unit (u) be renamed the dalton (Da). The definition of the unit would remain unchanged as one-twelfth the mass of a neutral 12C atom in its ground state. The International Union of Pure and Applied Chemistry (IUPAC) proposed that both units, u and Da, should be allowed in official use. 

An advantage of the definition of mass unit is that the mass of an atom or molecule can be characterized by a full number - the mass number A, which is equal to the number of nucleons (see Equation 1). The mass of an atom or atomic mass (ma) can be calculated approximately by the following equation (Equation 6) ... 

The isotopic composition of the elements (including the exact atomic mass and the abundance of the isotopes), the atomic weights of elements, definitions and abbreviations are summarized in Appendix II. 

It now remains to consider how the various gif terms are expressed with respect to the atomic masses and the dimensions of the molecule. The procedure, although not difficult or profound, is definitely tedious it is explained in Wilson, Decius, and Cross and other specialized textbooks. Fortunately, it is possible to tabulate the most commonly used ones in the form of general expressions into which the specific parameters of any molecule may be inserted. Such a tabulation and directions for its use are given in Appendix VI. It is found that... 

Resolving Power (RP) A measurement of how effectively a mass analyzer can distinguish between two peaks at different, but similar m/z. Mathematically, the formula M/ AM is used, where M is the m/z value for one of the peaks and AM is the spacing, in unified atomic mass units, between the peaks. Most commonly, AM is the mass resolution, either via the 10% valley or FWHM definitions (see below). (Note that the definition used will affect the resolving power calculated.) Resolving power of 500-1000 approximately corresponds to unit resolution (e.g., at m/z 700 and FWHM resolution of 0.7, RP = 1000). 

In 1808 John Dalton proposed his atomic theory, which included the statement that when atoms of two or more elements combine to form a compound, they combine in a definite ratio by number of atoms and by mass. This is called the law of definite proportions. This provided a means to determine the mass of one atom relative to another. It was necessary to assign a mass to one element to find the mass of another element in a compound. Today we use the most common carbon isotope, assigned a mass of 12.00 atomic mass units (amu), as the basis for comparative weights of the atoms. 

Unusual are measurements for which a direct link to the mole is useful. We should probably not talk about traceability in that connection, because that term is defined as a relation between measured values. An acceptable chain of measurements for compound X of established purity, containing element E that has isotope E and that would establish a link to the mole, then would take one of the following general routes the amount of substance (X)->n(E)->n( E)-> (12C) or n(X)->n(E)-> (C)-> (12C). The ratio of atomic masses m( E)lm( 12C) is also involved in the definition, but that ratio is known with a negligible uncertainty compared with the other links in the chain. Clearly, only in a few instances will laboratories attempt to execute such a chain of measurements for a link to the SI unit. Is it fear that such a difficult process is involved in every chemical analysis that has kept so many chemists from using the mole as the way to express chemical measurement values Or is it just habit and the convenience of a balance that subconsciously links amount of substance to amount of mass ... 

In the future, it will end in the value of the Avogadro constant and the value 1 of the atomic mass unit u defined as the mass of 1/12th of the mass of the 12C atom. This will happen when the definition of the kg (now the mass of the prototype of the kilogram ) will have changed into the mass of a number of12C atoms, i.e. of the mass of NAj-m(12C). 1000/12) . 

For a value to be traceable it must be related to stated references. By definition and convention the stated references are taken to include SI [6] reference values (e.g., atomic mass values), reference materials (RMs), as well as primary, reference, and standard methods. It is sometimes stated that chemical measurements are traceable to the mole. This is an incomplete statement as chemical measurements are simultaneously traceable to a number of references, inter alia, the mole, kg, meter, etc. Whilst it is considered desirable to employ high level references, such as the SI, where feasible, this is not always necessary in terms of fit for purpose criteria. Neither is it possible to relate all types of analyte (fat, fiber, protein, pH, etc.) to the SI. The key issue is that the references should be stated and fit for purpose. 

Some chemists feel that the mole is an unnecessary SI unit as they make measurements in mass/mass or mass/ volume units, using ratio methods. The definition and the importance of the mole has been discussed elsewhere [8], and the distinction has been made between its importance as a concept, the importance of the related atomic mass values, and the lesser role of the mole as a unit for actually reporting results. A distinctive feature of the mole is the need to define the entity . This is an extra dimension compared with other SI units. For example, it is not necessary to ask, is this a mass when measuring the mass of an object, in the way that it is critical to ask, is this lead before attempting to measure the amount of lead. A mole measurement thus requires two issues to be addressed, namely identity and amount. It follows therefore that traceability claims must show unbroken chains covering both of these issues. It is because of the existence of a vast number of chemical species that it is necessary to clearly specify and separate the specified chemical entities from all other possible chemical entities prior to measurement. This leads to complex chemical measurement processes, with considerable attention to validation of the measurement method being required. 

The mole can be realized in a similar way but, of course, there are millions of different types of mole. It is more appropriate to speak of realizing a mole and this can be done by measuring a specified entity and making use of chemical stoichiometry and atomic mass values to relate the measured property to mass, as defined in the definition of the mole, i.e., Mx = Nx/An = m/ M, where Mx = number of moles of entity X Nx = number of entities of X An = Avogadro s number m = mass of X M = atomic mass of X. 

Other numerical values are exact by definition. For example, the atomic mass scale was established by fixing the mass of one atom of 12C as 12.000 Ou. As many more zeros could be added as desired. Other examples include the definition of the inch (1 in = 2.5400 cm) and the calorie (1 cal = 4.184 00 J). 

Transformation from Internal to Normal Co-ordinates.—It is an important property of the internal co-ordinates, in terms of which we wish to represent the force field, that they should be geometrically defined, i.e. their definition should be made only in terms of the internal distances between the atoms, and should in no way involve the atomic masses. This is necessary to ensure that the force constants are unchanged for different isotopic species. It is clear that... 

So the mass of the 12C atom is exactly 12 atomic mass units, by virtue of the definition. 

The mass m is usually measured in Daltons (Da) or atomic mass units (amu mass of one gC12 nucleus = 12.0 Da = 12.0 amus, by definition 1 Da = 1.660538782 x 10-27 kg). Mass spectrometers can resolve isotopes rather well, and they can measure them to high precision (sometimes to 1 part in 107, but not in most routine or commercial instruments). Their mass range can be huge, from 1 amu to 100 kDa (the wider ranges have lower resolution, but always below 1 Da). The sample sizes are of the order of micrograms to nanograms. 

If the elemental formula of the structural unit of a polymer is known, the molar mass per structural unit can be calculated directly by addition (=summation) of the atomic masses. The molar mass is the oldest Additive Function it is additive by definition, since it is based on a fundamental law of chemistry the law of "Conservation of mass". 

In Chapter 2, you learned that the mass of an atom is expressed in atomic mass units. Atomic mass units are a relative measure, defined by the mass of carbon-12. According to this definition, one atom of carbon-12 is assigned a mass of 12 u. Stated another way, 1 u = of the mass of one atom of carbon-12. 

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