Molecular shapes determination

Intramolecular bonds (with molecular shape) determine molecular polarity. Molecular polarity, in turn, establishes whether or not one molecule will attract another molecule. If a IA or VIIA intramolecular bond is ionic, the molecule is polar. If a IA or VIIA intramolecular bond is covalent, the molecule is nonpolar. Polar molecules bond to each other (intermolecular bonds) to form an endless variety of geometric shapes that resemble beautiful three-dimensional sculptures. (See Figure 7.1.)... 

In fact, the molecular shape determines only the local shape of the interface, i.e. the interfacial curvatures. This can be demonstrated using simple equations from differential geometry describing the area of parallel surfaces. (Parallel surfaces are discussed in section 1.13.) If we trace out surface patcties that are parallel to a patch of area [Pg.145]

Bond polarity and molecular shape determine molecular polarity, which is measured as a dipole moment. When bond polarities counterbalance each other, the molecule is nonpolar when they reinforce each other, even partially, the molecule is polar. 

The van der Waals radius determines the shortest distance over which intermolecu-iar forces operate it is aiways larger than the covalent radius. Intermolecular forces are much weaker than bonding (intramolecular) forces. Ion-dipole forces occur between ions and poiar molecules. Dipole-dipole forces occur between oppositely charged poles on polar molecules. Hydrogen bonding, a special type of dipole-dipole force, occurs when H bonded to N, O, or F is attracted to the lone pair of N, O, or F in another molecule. Electron clouds can be distorted (polarized) in an electric field. Dispersion (London) forces are instantaneous dipole-induced dipole forces that occur among all particles and increase with number of electrons (molar mass). Molecular shape determines the extent of contact between molecules and can be a factor in the strength of dispersion forces. 

Numerically, it is now a common practice to calculate within the dielectric continuum formulation but employing cavities of realistic molecular shape determined by the van der Waals surface of the solute. The method is based upon finite-difference solution of the Poisson-Boltzmann equation for the electrostatic potential with the appropriate boundary conditions [214, 238, 239]. An important outcome of such studies is that even in complex systems there exists a strong linear correlation between the calculated outer-sphere reorganization energy and the inverse donor-acceptor distance, as anticipated by the Marcus formulation (see Fig. 9.6). More... 

Hydrodynamic Volume. Pol5miers are often described in terms of hydro-dynamic volume (HDV) or that volume occupied by the solvated chain. HDV and molecular shape, determined from light scattering measurements, may be used along with chemical microstructure to predict rheological behavior. 

Molecular Shapes Molecular shapes determine many of the properties of compounds. Water s bent geometry, for example, causes it to be a liquid at room temperature instead of a gas. It is also the reason ice floats on water and snowflakes have hexagonal patterns. 

Bond polarity and molecular shape determine molecular polarity, which is measured as a dipole moment. 

Dispersion (London) forces are instantaneous dipole-induced dipole forces that occur among all particles and increase with number of electrons (molar mass). Molecular shape determines the extent of contact between molecules and can be a factor in the strength of dispersion forces. 

In Chapter 10, we saw that electronegativity differences determine whether bond dipoles exist in a molecule and that molecular shape determines whether bond dipoles cancel (nonpolar molecules) or combine to produce a resultant dipole moment (polar molecules). Thus,... 

The Exclusion Principle is fundamentally important in the theory of electronic structure it leads to the picture of electrons occupying distinct molecular orbitals. Molecular orbitals have well-defined energies and their shapes determine the bonding pattern of molecules. Without the Exclusion Principle, all electrons could occupy the same orbital. 

For spherical or ellipsoidal cavities, eq. (16.44) can be solved analytically, but for molecular shaped surfaces, it must be done numerically, typically by breaking it into smaller fractions which are assumed to have a constant a. Once o-(rs) is determined, the associated potential is added as an extra term to the Hamilton operator. 

Click Coached Problems for a self-study module on determining molecular shapes. 

There are three factors that seem to be particularly important in determining the magnitudes of van der Waals forces the number of electrons, the molecular size, and the molecular shape. These factors are effective both for elements and compounds, though greater variety is found for compounds. 

What Are the Key Ideas The central ideas of this chapter are, first, that electrostatic repulsions between electron pairs determine molecular shapes and, second, that chemical bonds can be discussed in terms of two quantum mechanical theories that describe the distribution of electrons in molecules. 

Recently, a symmetry rule for predicting stable molecular shapes has been developed by Pearson Salem and Bartell" . This rule is based on the second-order, or pseudo, Jahn-Teller effect and follows from the earlier work by Bader . According to the symmetry rule, the symmetries of the ground state and the lowest excited state determine which kind of nuclear motion occurs most easily in the ground state of a molecule. Pearson has shown that this approximation is justified in a large variety of inorganic and small organic molecules. 

The Lewis stmcture of a molecule shows how the valence electrons are distributed among the atoms. This gives a useful qualitative picture, but a more thorough understanding of chemistry requires more detailed descriptions of molecular bonding and molecular shapes. In particular, the three-dimensional structure of a molecule, which plays an essential role in determining chemical reactivity, is not shown directly by a Lewis structure. 

Having introduced methane and the tetrahedron, we now begin a systematic coverage of the VSEPR model and molecular shapes. The valence shell electron pair repulsion model assumes that electron-electron repulsion determines the arrangement of valence electrons around each inner atom. This is accomplished by positioning electron pairs as far apart as possible. Figure 9-12 shows the optimal arrangements for two electron pairs (linear),... 

Our approach to these molecules illustrates the general strategy for determining the electron group geometry and the molecular shape of each inner atom in a molecule. The process has four steps, beginning with the Lewis structure and ending with the molecular shape. 

Follow the four-step process described in the flowchart. Begin with the Lewis structure. Use this stracture to determine the steric number, which indicates the electron group geometry. Then take into account any lone pairs to deduce the molecular shape. 

The carbon atom in CO2 has two groups of electrons. Recall from our definition of a group that a double bond counts as one group of four electrons. Although each double bond includes four electrons, all four are concentrated between the nuclei. Remember also that the VSEPR model applies to electron groups, not specifically to electron pairs (despite the name of the model). It is the number of ligands and lone pairs, not the number of shared eiectrons, that determines the steric number and hence the molecular shape of an inner atom. 

Use the Lewis structure of CIF3 to determine the steric number of the chlorine atom. Obtain the molecular shape from the orbital geometry after placing lone pairs in appropriate positions. 

C09-0023. The fourth molecular shape arising from a steric number of 5 is represented by the triiodide anion I3. Determine the molecular geometry and draw a three-dimensional picture of the triiodide ion. 

Follow the usual procedure. Determine the Lewis stmcture, then use it to find the steric number for xenon and to deduce electron group geometry. Next, use the number of ligands to identify the molecular shape. 

This relatively small catalog of molecular shapes accounts for a remarkable number of molecules. Even complicated molecules such as proteins and other polymers have shapes that can be traced back to these relatively simple templates. The overall shape of a large molecule is a composite of the shapes associated with its inner atoms. The shape around each inner atom is determined by steric numbers and the number of lone pairs. 

C09-0036. Describe the roie that eiectricai forces piay in determining each of the following properties (a) bond iength (b) bond poiarity (c) bond angie and (d) molecular shape. 

C09-0085. Determine the molecular shape and the ideal bond angles of each of the following (a) SO2 (b)... 

Determine the Lewis structure and the molecular shapes of the carbon atoms of this molecule. Suggest a reason for the reactivity of C3 Hg. ... 

C09-0140. Determine the Lewis stmctures, electron group geometries, and molecular shapes of the following compounds, which contain odd numbers of valence electrons. 

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