Atomic mass defined

Explain why it is incorrect to say that atomic mass is the mass of any atom of an element. How is atomic mass defined ... 

The conventional atomic mass is provided for elements that have their atomic masses defined in terms of an atomic mass interval and may be used in situations when a representative value of the atomic mass is required. (See also atomic mass interval.)... 

The location of the crossing seam (or seam) for an X3 system is established from the requirement that /-ab = rec = r c, where j-ab, rec, and fAc are the interatomic distances. Since the goal are the the geometric properties produced by this seam, hyperspherical coordinates (p,0,atomic masses are equal, say iiiB = me, the seam is defined [5] by... 

Formulate the constraining material-balance equations, based on conservation of the total number of atoms of each element in a system comprised of w elements. Let subscript k identify a particular atom, and define Ai as the total number of atomic masses of the /cth element in the feed. Further, let a be the number of atoms of the /cth element present in each molecule of chemical species i. The material balance for element k is then... 

Suppose that the atomic mass of C-12 is taken to be 5.000 amu and that a mole is defined as the number of atoms in 5.000 kg of carbon-12. How many atoms would there be in one mole under these conditions Hint There are 6.022 X 1023 C atoms in 12.00 g of C-12.)... 

Because the masses of nuclides are so small, they are normally reported as a multiple of the atomic mass constant, ma (formerly atomic mass unit, amu). The atomic mass constant is defined as exactly V12 the mass of one atom of carbon-12 ... 

The weight of atoms and their constituents can be given in kilograms. A proton, for example, weighs 1.67 x 10 kilograms, but its weight or mass can be expressed more conveniently in a measure called the atomic mass unit (amu). One amu is defined as 1/12 the mass of a carbon atom that consists of six protons. 

A glance at the periodic table (which will be covered in detail in Chapter 5) shows a list of elements with numbers that are not as neat as those for carbon. Iron, for instance, has an atomic mass of 55.845. Could an atom have a fractional proton or neutron Of course not. An element must have a fixed number of protons. That is what defines it as an element. However, the number of neutrons in the nucleus of an element can vary. Carbon, for instance, has two prominent forms. Carbon 12 has 6 protons and 6 neutrons whereas carbon 14 has 6 protons and 8 neutrons. 

The formula weight of a substance is equal to its number of grams per mole. Avogadro s number is the number of atomic mass units in 1 g. It is defined in that manner so that the atomic weight of an element (in amu) is numerically equal to the number of grams of the element per mole. Consider helium, with atomic weight 4.0 ... 

The actual calculation consists of minimizing the intramolecular potential energy, or steric energy, as a function of the nuclear coordinates. The potential-energy expressions derive from the force-field concept that features in vibrational spectroscopic analysis according to the G-F-matrix formalism [111]. The G-matrix contains as elements atomic masses suitably reduced to match the internal displacement coordinates (matrix D) in defining the vibrational kinetic energy T of a molecule ... 

MS equipment is evaluated on several performance metrics. Mass accuracy, mass resolution, and mass range are standard parameters frequently assessed to determine the suitability of an instrument. Mass accuracy is defined as the extent to which a mass analyzer reflects true m/z values and is measured in atomic mass units (amu), parts per million (ppm), or percent accuracy. 

Unified Atomic Mass Unit (u) A non-SI unit of mass defined as one twelfth of the mass of one atom of 12C in its ground state and 1.66 x 10-27 kg. The term atomic mass unit (amu) is not recommended to use since it is ambiguous. It has been used to denote atomic masses measured relative to a single atom of 160, or to the isotope-averaged mass of an oxygen atom, or to a single atom of 12C. 

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

Only a few relevant points about the atomic structures are summarized in the following. Table 4.1 collects basic data about the fundamental physical constants of the atomic constituents. Neutrons (Jn) and protons (ip), tightly bound in the nucleus, have nearly equal masses. The number of protons, that is the atomic number (Z), defines the electric charge of the nucleus. The number of neutrons (N), together with that of protons (A = N + Z) represents the atomic mass number of the species (of the nuclide). An element consists of all the atoms having the same value of Z, that is, the same position in the Periodic Table (Moseley 1913). The different isotopes of an element have the same value of Z but differ in the number of neutrons in their nuclei and therefore in their atomic masses. In a neutral atom the electronic envelope contains Z electrons. The charge of an electron (e ) is equal in size but of opposite sign to that of a proton (the mass ratio, mfmp) is about 1/1836.1527). 

The unified atomic mass unit (u), previously symbolized as AMU or amu, is defined to be 1/12 of the mass of one atom of isotope 12 of carbon. Therefore,... 

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. 

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. 

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