Molecules with Lone Pairs on the Central Atom

Molecules with Lone Pairs on the Central Atom... 

Optimize bond angles for molecules with lone pairs on the central atom(s) with the concept in mind that repulsions are in the order... 

We are now ready to study the structures of some simple molecules with lone pairs on the central atom. In this and subsequent sections, we use generalized chemical formulas in which A represents the central atom, B represents an atom bonded to A, and U represents a lone (unshared) pair on the central atom A. For instance, AB3U would represent any molecule with three B atoms bonded to a central atom A, with one lone pair on A. 

Molecules with Lone Pairs on the Central Atom VSEPR method for molecules with lone pairs m 0)... 

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. 

EXAMPLE 3.1 Predicting the shape of a molecule with no lone pairs on the central atom... 

Lone pairs on the central atom of a molecule also affect the shape of the molecule. To help us predict the shapes of molecules, we use a generic VSEPR formula AX Em to identify the different combinations of atoms and lone pairs attached to the central atom. We let A represent a central atom, X an attached atom, and E a lone pair. For example, the BF3 molecule, with three attached fluorine atoms and no lone pairs on B, is an example of an AX3 species. The sulfite ion, S()32- (18), is an example of an AX3E species. 

I Any molecule with one or more lone pairs on the central atom will have a different shape of the electron groups from the shape of the molecule (defined by the locations of the atoms). 

In nonlinear molecules that obey the octet rule, whenever two or more pairs of electrons are localized as a result of their attractive interactions with the nuclei, the a-spin and jff-spin tetrahedra will be brought into coincidence with each other, localizing any nonbonding electrons as lone pairs on the central atom. 

When there are two electron groups around the central atom, it is sp hybridized. AB2 molecules and ions with no lone pairs on the central atom have linear electronic geometry, linear molecular geometry, and sp hybridization on the central atom. 

A A In what two major ways does the presence of lone pairs of valence electrons affect the polarity of a molecule Describe two molecules for which the presence of lone pairs on the central atom helps to make the molecules polar. Can you think of a bonding arrangement that has lone pairs of valence electrons on the central atom but that is nonpolar Describe a molecule with this arrangement. 

Although it is somewhat more complicated, a similar analysis can be done to show that all x, y, and z coordinates sum to zero when there are four identical polar bonds arranged in a tetrahedron about a central atom. In fact, any time there are identical bonds symmetrically distributed around a central atom, with no lone pairs on the central atom, the molecule will be nonpolar overall, even if the bonds themselves are polar. 

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