Molecular geometry central atom

Molecular geometry (central atom has no lone pairs) 314... 

Molecular geometries for molecules with two to six electron-pair bonds around a central atom (A). 

Table 7.3 summarizes the molecular geometries of species in which a central atom is surrounded by two, three, or four electron pairs. The table is organized in terms of the number of terminal atoms, X, and unshared pairs, E, surrounding the central atom, A. 

Molecular geometries for molecules with expanded octets and unshared electron pairs. The gray spheres represent terminal atoms (X), and the open ellipses represent unshared electron pairs (E). For example. AX4E represents a molecule in which the central atom is surrounded by four covalent bonds and one unshared electron pair. 

Ihere are two criteria for determining the polarity of a molecule bond polarity and molecular geometry. If the polar A—X bonds in a molecule AXmE are arranged symmetrically around the central atom A, the molecule is nonpolar. 

Expanded octet More than four electron pairs about a central atom, 173-174 and hybridization, 187 and molecular geometry, 181 Expansion, 339-340... 

VSEPR model Valence Shell Electron Pair Repulsion model, used to predict molecular geometry states that electron pairs around a central atom tend to be as far apart as possible, 180-182... 

The molecular or ionic geometry is identical to its electronic geometry when there are no lone pairs on the central atom. 

The Lewis formula predicts 2 electron groups around the central Be atom and a linear electronic geometry. There are no lone pairs on the Cd atom, so the molecular geometry is the same as the electronic geometry linear (Section 8-5). 

The Lewis formula for the molecule (type AB4) predicts 4 electron groups around the central Sn atom and a tetrahedral electronic geometry. Since there are no lone pairs on Sn, the molecular geometry is also tetrahedral (Section 8-7). 

The Lewis formula shows 5 electron groups around the central S atom and its electronic geometry is trigonal bipyramidal. The molecular geometry is a seesaw due to the presence of 1 lone pair of electrons on the central S atom. 

The Lewis formula for the molecule (type AB2U) predicts 3 electron groups around the central N atom including 1 lone pair of electrons. The electronic geometry is trigonal planar and the molecular geometry is angular or bent (Table 8-3). 

Using Valence Bond (VB) theory, the central atoms of the molecules with formulas AB2U2 and AB3U should undergo sp3 hybridized with predicted bond angles of 109.5°. If no hybridization occurs, bonds would be formed by the use of p orbitals. Since the p orbitals are oriented at 90° from each other, the bond angles would be 90°. Note that hybridization is only invoked if the actual molecular geometry data indicate that it is necessary. 

The Lewis dot formula predicts 4 regions of high electron density around the central N atom, a tetrahedral electronic geometry and a pyramidal molecular geometry. The N atom has sp3 hybridization (Sections 8-8 and 28-14). The three-dimensional structure is shown below. 

In PF6 there are 5 + (6 x 7) +1 = 48 valence electrons or 24 electron pairs. A plausible Lewis structure follows. Since there are six atoms and no lone pairs bonded to the central atom, the electron-group geometry and molecular shape are octahedral. 

The VSEPR notation for the Cl2F+ ion is AX2E3. According to Table 11.1, molecules of this type exhibit an angular molecular geometry. Our next task is to select a hybridization scheme that is consistent with the predicted shape. It turns out that the only way we can end up with a tetrahedral array of electron groups is if the central chlorine atom is sp3 hybridized. In this scheme, two of the sp3 hybrid orbitals are filled, while the remaining two are half occupied. 

The Cl—F and Cl—Cl bonds in the cation are then formed by the overlap of the half-filled sp3 hybrid orbitals of the central chlorine atom with the half-filled p-orbitals of the terminal Cl and F atoms. Thus, by using sp3 hybridization, we end up with the same bent molecular geometry for the ion as that predicted by VSEPR theory (when the lone pairs on the central atom are ignored)... 

This approach, also often called the stockholder scheme, was introduced in 1977 by Hirshfeld [22]. The central idea of the Hirshfeld method originates in x-ray crystallography. It proposes to divide the electron density among the atoms in a molecule, guided by a promolecular density. More precisely, once a molecular geometry is known, a promolecular density p°(r) is composed by simply summing the density of each atom A (denoted p°A(r)) in an isolated state ... 

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