Polyatomic ions defined

By means of mass spectrometry, the mass of atoms and molecules, via mass-separated charged atomic or polyatomic ions, can be determined by measuring the mass-to-charge ratio m/z), whereby the mass of an atom or a molecule is not measured in g or kg, but in a multiple of the atomic mass constant mu (atomic mass unit). The atomic mass unit mu is defined as one-twelfth the mass of a neutral 12C atom, ma (12C), in its ground state ... 

Molecular rotation In a normal crystal every atom occupies a precise mean position, about which it vibrates to a degree depending on the temperature molecules or polyatomic ions have precisely defined orientations as well as precise mean positions. When such a crystal is heated, the amplitude of the thermal vibrations of the atoms increases with the temperature until a point is reached at which the regular structure breaks down, that is, the crystal melts. But in a few types of crystal it appears that notation of molecules or polyatomic... 

The oxidation number, also known as the oxidation state, of an atom in a compound (or in a free element or polyatomic ion) is defined as the number of electrons possessed by a free (an uncombined) atom of the element— Nf ee—minus the number that is assigned to the atom in the compound (or element or ion)—... 

Oxidation numbers (also called oxidation states) are used as a sort of bookkeeping method for keeping track of the electrons in polyatomic ions or compounds that have covalent bonds. (For monatomic ions, the charge on the ions works just as well.) Oxidation number is defined as the number of electrons in a free atom minus the number controlled by that atom in the compound. The control of electrons in a covalent bond is assigned to the more electronegative atom of the bond. Eight simple rules can be used to determine the oxidation number of an element from the formula of its compound or ion (Section 16.1). 

A molecular multipole expansion is poorly converged at distances of chemical interest, for instance, at distances between atoms of molecules in a dimer or crystal. To obtain electrostatic interaction energy between heteronuclear diatomic molecules at close distance it is always better to model observed molecular point dipoles by a distributed dipole model that places net charges on the atoms. In general, a distributed multipole model of a molecule consists of a set of sites with each site having its own multipoles. Obviously distributed multipole models are not uniquely defined. For instance, at which site in a polyatomic ion is the ionic charge (monopole) located Do you spread it around evenly among the sites ... 

Because of the importance of controlling the internal energy of the ions, the ion source is a critical feature of our instrument when used for the study of polyatomic ions. The use of flow-tube technology ensures that the ion energy is well defined. Our flow tube (Schultz and Armentrout, 1991a) can be varied in... 

The diagram below shows the Lewis structure for the polyatomic ion nitrate (NOs"). Define the term polyatomic ion, and give examples of other ions of this type. 

Energy characteristics of atoms also define, to a large extent, the strengths of their bonds in molecules, polyatomic ions and radicals. The work required to disrupt a chemical bond, e.g. to separate chemically bonded atoms from the equilibrium distance to a practically infinite one (in the ground state) is called bond energy Eb). In case of the A2 and AX molecules, Eb is equal to the dissociation energy of the molecule (De) which can be determined by thermochemical, calorimetric, kinetic, mass-spectroscopic and molecular spectroscopic techniques. By definition, De characterizes atoms in molecules at the equilibrium state with zero-point energy,... 

Oxidation numbers, sometimes called oxidation states, are signed numbers given to atoms in molecules and ions to define their positive or negative character. The oxidation number refers to the number of formal charges that an atom would have in a molecule or polyatomic ion in the case of electrons being completely transferred in the direction indicated by the electronegativity difference between atoms. 

Figure 3.5 Use of three-isotope plots to check for spectral interferences in MC-ICP-MS. Each point represents the mean of an isotope ratio measurement of a standard (filled circles) or a sample (empty circle) of natural isotopic composition. Isotope ratios are plotted on the delta scale (5) as relative deviations in parts per thousand from the known isotope ratio of an isotopic reference material of natural isotopic composition. The diagonal line represents the theoretical fractionation curve as defined by the isotopic masses and an exponential fractionation law. (a) Absence of isobaric interferences. Data points from standard and sample plot on the theoretical curve, (b) At least one isotopic signal in the mass spectrum of the standard and the sample is subject to spectral interference from an isobaric nuclide, polyatomic ion, or doubly charged ion. (c) Matrix differences between sample and standard result in an offset of the sample data points from the theoretical fractionation curve.

In certain cases, it is possible to obtain the local mole fractions of the solvent components around the ion from measurements of a spectroscopic property p of the ion that is sensitive to its surroundings. The property p may be the wave number v of light absorption of a transition metal ion or of the vibrational mode of a polyatomic ion or the chemical shift 6 of an NMR signal, etc. Then the local mole fraction of solvent component A around the ion P is measured (in fact, defined) as ... 

Figure 8.12 shows evaluated (ADO) cross section of collisions of Ar+ with H2, CH4 and NH3 as a function of kinetic energy. At low energy, the interaction is defined by charge-dipole interaction and the cross section is many times larger than hard-sphere cross section. Under these conditions, the difference in collision cross sections of atomic and polyatomic ions of the same charge and mass is diminished. 

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