Electron pairs Lone pairs

Lone-pair electrons (Section 1.4) Nonbonding valence-shell electron pairs. Lone-pair electrons are used by nucleophiles in their reactions with electrophiles. 

A nucleophile, a species with an unshared electron pair (lone-pair electrons),... 

While the present discussions focus on orbital hybridization relative to bonds between atoms, it is important to recognize that nonbonding electron pairs (lone pairs) also participate in orbital hybridization. Thus, as illustrated in Figure 5.4 and relating to. sp3-hybridized centers, for the purposes of determining orbital hybridization, lone pairs can be treated as bonds between a central atom and nothing. 

The localized character of the geminals can be displayed by introducing the concepts of charge centroids [22] and charge ellipsoids [23, 24] of the geminal one-electron densities. By using these localization measures, we can classify the geminals as core electron pairs, lone pairs and bond pairs [25]. 

We are now ready to study the structures of some simple molecules with unshared valence electron pairs (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 an unshared valence shell electron pair (lone 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 unshared valence pair on A. 

What is the most stable three-dimensional arrangement of four electron pairs (lone pairs or bonding pairs) attached to a central atom Three electron pairs ... 

Electron groups Electron group arrangement Bond electron pairs Lone pairs Geometry of molecule or composite ion Example... 

The idea that covalent bonding can be predicted by achieving noble gas electron configurations for all atoms is a simple and very successful idea. The rules we have used for Lewis structures describe correctly the bonding in most molecules. However, with such a simple model, we should expect some exceptions. Boron, for example, tends to form compounds in which the boron atom has fewer than eight electrons around it—that is, it does not have a complete octet. Boron trifluoride, BF3, a gas at normal temperatures and pressures, reacts very energetically with molecules such as water and ammonia that have unshared electron pairs (lone pairs). 

This means that the bonding and nonbonding electron pairs (lone pairs) around a given atom are positioned as far apart as possible. 

BONDING ELECTRON PAIRS LONE PAIRS GEOMETRY... 

Arrow formalism, curved arrow formalism, or electron pushing (Section 1.4) A mapping device for chemical reactions. The electron pairs (lone pairs or bond pairs) are pushed using curved arrows that show the bonds that are forming and breaking in the reaction. 

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