Linear molecules VSEPR model

Multiple bonds are treated as a single unit m the VSEPR model Formaldehyde is a trigonal planar molecule m which the electrons of the double bond and those of the two single bonds are maximally separated A linear arrangement of atoms m carbon diox ide allows the electrons m one double bond to be as far away as possible from the elec Irons m the other double bond... 

Valence shell electron pair repulsion (VSEPR) model (Section 110) Method for predicting the shape of a molecule based on the notion that electron pairs surrounding a central atom repel one another Four electron pairs will arrange them selves in a tetrahedral geometry three will assume a trigo nal planar geometry and two electron pairs will adopt a linear arrangement... 

VSEPR model, the dihalides of Be and Mg and the heavier halides of Ca and Sr are essentially linear. However, the other dihalides are appreciably bent, e.g. Cap2 145°, Srp2 -- 120°, Bap2 108° SrCl2 - 130°, BaCh - 115° BaBri -115° Bah 105°. The uncertainties on these bond angles are often quite large ( 10°) and the molecules are rather flexible, but there seems little doubt that the equilibrium geometry is substantially non-linear. This has been interpreted in terms of sd (rather than sp) hybridization or by a suitable id hoc modification of the VSEPR theory. ... 

A molecule with only two atoms attached to the central atom is BeCl2. The Lewis structure is CI — Be — CE, and there are no lone pairs on the central atom. To be as far apart as possible, the two bonding pairs lie on opposite sides of the Be atom, and so the electron arrangement is linear. Because a Cl atom is attached by each bonding pair, the VSEPR model predicts a linear shape for the BeCL molecule, with a bond angle of 180° (4). That shape is confirmed by experiment. 

For AX molecules with no lone pairs in the valence shell of A, both the VSEPR model and the LCP model predict the same geometries, namely AX2 linear, AX3 equilateral triangular, AX4 tetrahedral, AX5 trigonal bipyramidal, and AX octahedral. Indeed Bent s tangent sphere model can be used equally as a model of the packing of spherical electron pair domains and as a model of the close packing of spherical ligands around the core of the central atom. 

The VSEPR model predicts that CO2 is a linear molecule. 

Thus CO2 has two "effective pairs" that lead to its linear structure, whereas O3 has three "effective pairs" that lead to its bent structure with a 120° bond angle. Therefore, to use the VSEPR model for molecules (or ions) that have double bonds, we use the same tools as those given on p. 430, but we count any double bond the same as a single electron pair. Although we have not shown it here, triple bonds also count as one repulsive unit in applying the VSEPR model. 

The valence shell of Sr in Srp2 contains two bonding pairs of electrons and no lone pairs, therefore, by the VSEPR model Srp2 should be a linear molecule. 

Finally we turn our attention to the suboxide. Experiments show that the atoms are connected in the sequence OCCCO. The Lewis structure of is presumably 0 C C C 0 . The observed C-C and C-O bond distances are indeed consistent with a double bond description 0=C=C=C=0. The VSEPR model then leads to the prediction that the molecule is linear. This is not strictly true the molecule is bent at the central atom with ZCCC = 156° The potential energy maximum at ZCCC = 180° (see Fig. 9.1) is, however, less than 0.25 kJ mol above the minimum, and the molecule is therefore often described as pseudo linear [1]. Why the small energy hump at ZCCC = 180° is there, nobody seems to know. 

Using the VSEPR model 000 (Section 9.2), we conclude that the molecule is linear and therefore has no dipole moment, aoo (Section 9.3) Thus, there are no dipole-dipole forces. Only dispersion forces operate between the CS2 molecules. 

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