Atomic mass modern system

The mass number gives the total number of protons and neutrons in an atom of an element, but it does not convey the absolute mass of the atom. To work with the masses of elements, we use comparative masses. Initially, Dalton and the other pioneers of the atomic theory used the lightest element hydrogen and compared masses of other elements to hydrogen. The modern system uses C-12 as the standard and defines one atomic mass unit (amu) as 1/12 the mass of one C-12 atom. One amu is approximately 1.66 X 10 g. This standard means the masses of individual protons and neutrons are slightly more than 1 amu as shown in Table 4.6. 

From the isotopic decomposition of normal He one finds that the mass-4 isotope, 4He, is 99.986% of all helium. It is the second most abundant nucleus in the universe Modern observations of the interstellar gas reveal it to be 10.3 times less abundant than hydrogen. The elemental abundance is He = 2.72 x 109 per million silicon atoms in solar-system matter. 

As we saw in Chapter 2, the first quantitative information about atomic masses came from the work of Dalton, Gay-Lussac, Lavoisier, Avogadro, Cannizzaro, and Berzelius. By observing the proportions in which elements combine to form various compounds, nineteenth-century chemists calculated relative atomic masses. The modern system of atomic masses, instituted in 1961, is based on 12C (carbon-12) as the standard. In this system I2C is assigned a mass of exactly 12 atomic mass units (amu), and the masses of all other atoms are given relative to this standard. 

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. 

Development of new MS systems focuses on the ability to measure smaller samples accurately, either for greater sensitivity or shorter-lived radionuclides. Advances in both construction and design have been applied, with the effect that the modern MS operator can measure ever-smaller numbers of atoms. A related development is the ability to individuate those samples more thoroughly by increasing MS resolution to decrease interference from isobars—atoms and molecules with the same atomic mass number but with minute differences in mass. 

The most widely regarded approach to accomplish the determination of as many pesticides as possible in as few steps as possible is to use MS detection. MS is considered a universally selective detection method because MS detects all compounds independently of elemental composition and further separates the signal into mass spectral scans to provide a high degree of selectivity. Unlike GC with selective detectors, or even atomic emission detection (AED), GC/MS may provide acceptable confirmation of the identity of analytes without the need for further information. This reduces the need to re-inject a sample into a separate GC system (usually GC/MS) for pesticide confirmation. Through the use of selected ion monitoring (SIM), efficient ion-trap or quadrupole devices, and/or tandem mass spectrometry (MS/MS), modern GC/MS instruments provide LODs similar to or lower than those of selective detectors, depending on the analytes, methods, and detectors. 

At the core of any science is measurement. Being able to measure volumes, pressures, masses, and temperatures as well as the ability to count atoms and molecules allows chemists to understand nature more precisely. Modern science uses the International System of Units (SI) that was adopted worldwide in 1960. The metric system of measurement, which is consistent with the International System, is widely used in chemistry and is the principal system used in this book. 

Thermodynamics was developed mostly in the nineteenth century. This was after the acceptance of the modern atomic theory of Dalton but before the ideas of quantum mechanics (which imply that the microscopic universe of atoms and electrons follow different rules than the macroscopic world of large masses). Therefore, thermodynamics mostly deals with large collections of atoms and molecules. The laws of thermodynamics are macroscopic rules. Later in the text, we will cover microscopic rules (that is, quantum mechanics), but for now remember that thermodynamics deals with systems we can see, feel, weigh, and manipulate with our own hands. 

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