Non-bonding pairs

The theory as presented so far is clearly incomplete. The topology of the density, while recovering the concepts of atoms, bonds and structure, gives no indication of the localized bonded and non-bonded pairs of electrons of the Lewis model of structure and reactivity, a model secondary in importance only to the atomic model. The Lewis model is concerned with the pairing of electrons, information contained in the electron pair density and not in the density itself. Remarkably enough however, the essential information about the spatial pairing of electrons is contained in the Laplacian of the electron density, the sum of the three second derivatives of the density at each point in space, the quantity V2p(r) [44]. 

Other chemists show the bonding pairs as lines between atoms. Dots are reserved for representing a lone pair (a non-bonding pair) of electrons. 

The fundamental principle of the Valence-Shell Electron-Pair Repnlsion theory is that the bonding pairs and lone, non-bonding pairs of electrons in the valence level of an atom repel one another. As you know, electron pairs of atoms are localized in orbitals, which are shapes that describe the space in which electrons are most likely to be found around a nucleus. 

This structure gives stability to the layers. The fourth, non-bonding pair of electrons is delocalized and free to move within the layers. This gives graphite the ability to conduct an electric current. There are no intramolecular forces between the layers. Dispersion forces attract one layer to another, enabling layers to slide by one another. Graphite feels slippery as a result of this characteristic. In fact, many industrial processes use graphite as a lubricant. 

Form A is a transition metal hydride while in form B the silicon hydride behaves as a two electron ligand. The electrons donated by the ligand to the transition metal happen to be a bonding pair in H-SiRj instead of a non-bonding pair in... 

The shape of a molecule or ion is governed by the shape adopted by its constituent atoms. In PHj, for example, there are four electron pairs, but three of them are bonded pairs and one is a non-bonded pair. The four electron pairs adopt a tetrahedral shape but the three bonded pairs adopt a pyramidal shape. So the PHj molecule is described as pyramidal, not tetrahedral. As the base of this pyramidal structure is triangular rather than, say, square, the shape is more correctly referred to as trigonal pyramidal. 

Boron is in Group 3 and so has three electrons in the outer shell. The three Cl atoms contribute one electron each, giving a total of six electrons involved in bonding. So there are three B-Cl bonds and no non-bonding pairs on the boron atom. The shape of the boron trichloride molecule will be trigonal (or trigonal planar) with all four atoms in the same plane. 

Four pairs of electrons (two bonding pairs and two non-bonding pairs) H O... 

In water, the action of the two non-bonding pairs reduces the bond angle to 104-5°. 

To predict the shape of a molecule, you first calculate the number of electron pairs and their arrangement. However, to obtain the actual molecular shape, you must also take into account whether these electron pairs are bonding or non-bonding pairs. 

Ligands must have at least one non-bonding pair (lone pair) of electrons and these are shown as a pair of dots on the atoms. 

They have non-bonding pairs of electrons on the oxygen and nitrogen atoms, respectively. 

Amines, like ammonia, are weak bases (see p. 31) and dissociate to a slight extent in aqueous solution. In the reaction, the non-bonded pair (lone pair) of electrons on the nitrogen atom in the amine molecule accepts a proton (hydrogen ion) from the water molecule, thus generating alkylammonium ions and hydroxide ions. The latter ions make the solution alkaline. For example CHjNH aq) H20(l) CH3NH3 (aq) -r OH (aq)... 

The direct electrochemical oxidation of alcohols involves removal of one electron from a non-bonding pair on oxygen. Relatively anodic potentials are required and the use of reagents, which can provide another mechanism for the oxidation step, has been extensively explored. Electrochemistry is then involved in the reoxidation of spent reagent and often the system can be adapted so as to require only a catalytic amount of reagent. 

Q The valence shell of the central atom of a molecule possesses two bonding and three non-bonding pairs of electrons. State, with reasoning, the shapes of (i) the electron pair distribution and (ii) the molecule. 

The alternative approach is to place the negative charge on the central iodine atom so that it has the xenon configuration, 5s25p6. The divalent state of I is produced by promoting one of the 5p electrons to a 5d orbital and this results in the same conclusion from the two bonding pairs and three non-bonding pairs as the previous treatment. 

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