Atomic and Molecular Masses

Nominal ion mass the mass of the ion for a given empirical formula, calculated by adding the integer mass of the most abundant isotope of each element (e.g., = 1 and = 12). 

Most abundant ion mass the mass that corresponds to the most abundant peak in the isotopic cluster of the ion of a given empirical formula. Average mass the mass of an ion for a given empirical formula calculated with the atomic weight of each element (e.g., C = 12.01115 and H = 1.00797) that is, the average of the isotopic masses of each element, weighted for isotopic abundance. The average mass represents the centroid of the distribution of the isotopic peaks of the molecular ion and is used by chemists in stoichiometric calculations. 

The monoisotopic mass is meaningful for low-mass compounds because the elemental composition can be determined from a well-defined isotopic pattern of the molecular ion. The nominal and monoisotopic masses can both be correlated with the most abundant peak in the isotopic cluster. As the mass of a compound, however, increases, the isotopic pattern becomes more synunetrical and extends over many masses [21]. Also, the monoisotopic peak becomes difficult to identify. For high-mass compounds (e.g., proteins and oligonucleotides), the molecular ion profile measured coalesces and becomes a single asymmetric peak. For such compounds, the average mass value is accepted as the molecular mass. 

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Relative isotopic, atomic and molecular masses are measured on a scale in which the mass of an atom of carbon-12 is exactly 12 atomic mass units (a.m.u.). 

Mass spectrometry is an analytical technique to measure molecular masses and to elucidate the structure of molecules by recording the products of their ionization. The mass spectrum is a unique characteristic of a compound. In general it contains information on the molecular mass of an analyte and the masses of its structural fragments. An ion with the heaviest mass in the spectrum is called a molecular ion and represents the molecular mass of the analyte. Because atomic and molecular masses are simple and well-known parameters, a mass spectrum is much easier to understand and interpret than nuclear magnetic resonance (NMR), infrared (IR), ultraviolet (UV), or other types of spectra obtained with various physicochemical methods. Mass spectra are represented in graphic or table format (Fig. 5.1). 

From Equation 4.79, it is then recognized that the isotope effect is given by a symmetry number factor and terms which depend only on the normal mode vibrational frequencies. There are no terms in the equality that depend explicitly on atomic and molecular masses or on moments of inertia. 

Atomic and molecular masses are assigned relative to the mass of the carbon isotope, 12C, whose atomic weight is defined as exactly 12. 

The most common method of determining both atomic and molecular masses is with an instrument called a mass spectrometer, shown schematically in Figure 3.10. The sample is first vaporized and is then injected as a dilute gas into... 

CHAPTER 2 Atomic and Molecular Mass Molar Mass 16... 

While most beginning chemistry students associate the name of Avogadro with moles and Avogadro s number, Amedeo Avogadro s principal contribution to the chemical sciences is the hypothesis that, at equal temperatures and pressures, equal volumes of gas contain equal numbers of particles. This work made the determination of several atomic and molecular masses possible. In other words, the volume of a gas is directly proportional to the number of gas molecules, as long as temperature and pressure are held constant. 

When a pure elemental gas, such as neon, was analyzed by a mass spectrometer, multiple peaks (two in the case of neon) were observed (see Fig. 1.11). Apparently, several kinds of atoms of the same element exist, differing only by their relative masses. Experiments on radioactive decay showed no differences in the chemical properties of these different forms of each element, so they all occupy the same place in the periodic table of the elements (see Chapter 3). Thus the different forms were named isotopes. Isotopes are identified by the chemical symbol for the element with a numerical superscript on the left side to specify the measured relative mass, for example °Ne and Ne. Although the existence of isotopes of the elements had been inferred from studies of the radioactive decay paths of uranium and other heavy elements, mass spectrometry provided confirmation of their existence and their physical characterization. Later, we discuss the properties of the elementary particles that account for the mass differences of isotopes. Here, we discuss mass spectrometry as a tool for measuring atomic and molecular masses and the development of the modern atomic mass scale. 

I hapter 1 explained how chemical and physical methods are used to estab-lish chemical formulas and relative atomic and molecular masses. This chapter begins our study of chemical reactions. We start by developing the concept of the mole, which allows us to count molecules by weighing macroscopic quantities of matter. We examine the balanced chemical equations that summarize these reactions and show how to relate the masses of substances consumed to the masses of substances produced. This is an immensely practical and important subject. The questions how much of a substance will react with a given amount of another substance and how much product will be generated are central to all chemical processes, whether industrial, geological, or biological. 

Atomic and Molecular Masses, Mole Concept and Chemical Equation... 

Some people think that Amedeo Avogadro (1776-1856) determined the number of particles in a mole and that is why the quantity is known as Avogadro s number. In reafity Avogadro built a theoretical foundation for determining accurate atomic and molecular masses. The concept of a mole did not even exist in Avogadro s time. 

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