Polyatomic units

Many elements can give rise to more than one elementary substance. These may be substances containing assemblages of the same mono- or poly-atomic unit but arranged differently in the solid state (as with tin), or they may be assemblages of different polyatomic units (as with carbon, which forms diamond, graphite and the fullerenes, and with sulfur and oxygen). These different forms of the element are referred to as allotropes. Their common nomenclature is essentially trivial, but attempts have been made to develop systematic nomenclatures, especially for crystalline materials. These attempts are not wholly satisfactory. 

The terms molecular and non-molecular describe geometrical structure, namely the presence or absence of discrete polyatomic units separated by non-bonding boundaries. The infinite linkages characteristic of structural non-molecularity can be classified as one-, two- or three-dimensional the first two types correspond to chain and layer structures, respectively. Structurally molecular complexes may have any charge or size. 

In order to apply this model to polyatomic solute/solvent systems a procedure for determining the effective diatomic(s) corresponding to the polyatomic solute of interest must be established. In the original work of Schweizer and Chandler (25), polyatomics were treated as a superposition of isolated diatomic bonds. This model completely ignores any shielding of solvent collisions by neighboring bonds in a polyatomic solute. Later studies have modeled the entire polyatomic solute as psuedo-diatomic whose "atoms represents polyatomic units on either side of the bond of interest (25,38). This approach, which is the one we adopt here, is believed to more realistically represent the solvent induced forces along a bond in a polyatomic molecule. 

When the proton is attached to a mobile ion, two modes of transport have been proposed. (1) Polyatomic ions like H3O+ or NH4+ may migrate by a simple jump from site to site in the lattice, as has been claimed in the jS-aluminas and in the zeolites.(2) A more involved mechanism based on the simultaneous difiiision of two types of polyatomic units, the so-called vehicular mechanism, was suggested... 

Crystals containing polyatomic units of structure which are effectively non-spherical usually exhibit some increase in disorder in the mutual orientation of these units when the crystal melts. For such crystal lattices, the total entropy of fusion, Sf, may be written as the sum of two terms Sf = Spos. + 5or.-... 

Both routes may also operate with polyatomic units containing an even number of carbon atoms, such as neutral or Ionized 63 and C4 units. At the low pressures of the Interstellar medium the addition of carbon... 

In the case of a polyatomic molecule, rotation can occur in three dimensions about the molecular center of mass. Any possible mode of rotation can be expressed as projections on the three mutually perpendicular axes, x, y, and z hence, three moments of inertia are necessar y to give the resistance to angular acceleration by any torque (twisting force) in a , y, and z space. In the MM3 output file, they are denoted IX, lY, and IZ and are given in the nonstandard units of grams square centimeters. 

Polyatomic Cations. Polyatomic cations derived by addition of more protons than required to give a neutral unit to polyatomic anions are named by adding the ending -onium to the root of the name of the anion element for example, PH4, phosphonium ion HjU, iodonium ion H3O+, oxonium ion CH3OHJ, methyl oxonium ion. 

Count the number of valence electrons. For a molecule, simply sum up the valence electrons of the atoms present. For a polyatomic anion, one electron is added for each unit of negative charge. For a polyatomic cation, a number of electrons equal to the positive charge must be subtracted. 

Polar molecule A molecule in which there is a separation of charge and hence positive and negative poles, 183-185 dipole force, 237 orientation, 183 Polarimeter, 600 Polarity, 184-185 Pollutants, 6 Polyamide, 615-616 Polyatomic ion, 36,39 Polyatomic molecules, 654 Polyester A large molecule made up of ester units, 614-615 Polyethylene, 611-612... 

For diatomic molecules, B0 is the rotational constant to use with equation (10.125), while Be applies to equation (10.124). They are related by Bq = Be 2 - The moment of inertia 70(kg-m2) is related to 50(cm ) through the relationship /0 = h/ 8 x 10 27r22 oc), with h and c expressed in SI units. For polyatomic molecules, /a, /b, and Iq are the moments of inertia to use with Table 10.4 where the rigid rotator approximation is assumed. For diatomic molecules, /0 is used with Table 10.4 to calculate values to which we add the anharmonicity and nonrigid rotator corrections. 

There are many different polyatomic anions, including several that are abundant in nature. Each is a stable chemical species that maintains its stmcture in the solid state and in aqueous solution. Polyatomic anions are treated as distinct units when writing chemical formulas, naming compounds, or drawing molecular pictures. The names, formulas, and charges of the more common polyatomic anions are listed in Table 3-4. You should memorize the common polyatomic ions because they appear regularly throughout this textbook. 

Note, however, that hydroxide is not on the periodic chart. That is because hydroxide is a polyatomic ion. Remember that polyatomic ions always travel together as a unit. The only way to know the charge of a polyatomic ion is to memorize it. Hydroxide has a charge of -1. So, when sodium (with a charge of +1) and a hydroxide come into contact with one another, a sodium atom gives its one valence electron to a hydroxide ion, which needs one... 

Polyatomic ion A charged group of atoms that react as a unit. 

Optionally, if a polyatomic ion is involved which does not change during the reaction, you may treat the whole thing as one unit, instead of considering the atoms which make it up. 

Ionic compounds are made up of positively charged ions (usually metal ions) and negatively charged ions (usually non-metal ions or polyatomic anions) held together by electrostatic forces of attraction. Molecular compounds are made up of discrete units called molecules. Generally they consist of a small number of nonmetal atoms held together by covalent bonds (sharing of electrons). 

Force Within Unit (Intra) covalent bond within polyatomic ion — — covalent bond... 

Hint One formula unit of the compound in part (c) contains two identical monatomic ions and one polyatomic ion. 

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