Linear arrangement VSEPR

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... 

These lowest oxidation states can be associated with a coordination number of two and a linear arrangement of the ligands about the central atom. On VSEPR theory these are 10-electron species, with a trigonal bipyramidal arrangement of three lone pairs of electrons in equatorial positions and two halogen ligands in axial positions. 

When two electron groups attached to a central atom are oriented as far apart as possible, they point in opposite directions. The linear arrangement of electron groups results in a molecule with a linear shape and a bond angle of 180°. Figure 10.4 shows the general form (top) and shape (middle) with VSEPR shape class (AXa), and the formulas of some linear molecules. 

There are two electron pairs in the valence shell for beryllium, and the VSEPR model predicts that they will have a linear arrangement (see Figure 10.2). Fluorine atoms are bonded in the same direction as the electron pairs. Hence, the geometry of the Bep2 molecule is linear—that is, the atoms are arranged in a straight line (see Figure 10.4). 

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. 

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),... 

According to VSEPR theory, the most stable arrangement of the three lone pairs of electrons would be in the equatorial position, as shown in (1), where they would be less crowded. Therefore, a linear structure is the correct molecular geometry of the molecule. 

The Lewis structure reveals a VSEPR number of 5 for the central iodine atom, two bonded neighbors and three unshared pairs. To determine which corners of the trigonal bipyramid are occupied by the terminal iodine atoms, find the arrangement which maximizes the angles between the unshared pairs. The preferred arrangement, Fig. 9-45(a), must be the one in which the unshared pairs are all at 120° because any other alternative [Fig. 9-45(b) and (c)] would have two sets of pairs at 90°. Therefore, the two terminal iodine atoms must occupy the axial positions (180° to each other), making the molecule linear. 

The VSEPR model for predicting structure does not work for complex ions. However, we can safely assume that a complex ion with a coordination number of 6 has an octahedral arrangement of ligands, and that complexes with two ligands are linear. On the other hand, complex ions with a coordination number of 4 can be either tetrahedral or square planar there is no reliable way to predict which will occur in a particular case. 

The VSEPR theory predicts the three-dimensional shapes of molecules. It is based on simple electrostatics—electron pairs in a molecule will arrange themselves in such a way as to minimize their mutual repulsion. The steric number determines the geometry of the electron pairs (linear, trigonal pyramidal, tetrahedral, and so forth), whereas the molecular geometry is determined by the arrangement of the nuclei and may be less symmetric than the geometry of the electron pairs. 

Valence shell electron pair repulsion theory places the two electron pairs on Be 180° apart, that is, with linear electronic geometry. Both electron pairs are bonding pairs, so VSEPR also predicts a linear atomic arrangement, or linear molecular geometry, for BeCl2. 

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