Lone-pair

When necessary a lone pair of electrons are considered a substituent and given a priority lower than H. 

Co-ordinate bonds are formed by the sharing of electrons, both electrons being donated by the same atom. Thus the hydrogen ion, has no outer electrons whilst ammonia has eight, six shared with hydrogen atoms and one lone-pair. This lone-pair is donated to the hydrogen ion and the ammonium ion is formed ... 

The shapes of covalent compounds are determined by the tendency for bonding pairs to be as far apart as possible whilst lone pairs have a greater effect than bonding pairs (VSEPR theory). 

Figure C2.14.2. The hydrogen bond in water. The oxygen lone pairs (shaded blobs) are the donors, and the hydrogen atoms the acceptors [ 177, 178].

Unlike the forces between ions which are electrostatic and without direction, covalent bonds are directed in space. For a simple molecule or covalently bonded ion made up of typical elements the shape is nearly always decided by the number of bonding electron pairs and the number of lone pairs (pairs of electrons not involved in bonding) around the central metal atom, which arrange themselves so as to be as far apart as possible because of electrostatic repulsion between the electron pairs. Table 2.8 shows the essential shape assumed by simple molecules or ions with one central atom X. Carbon is able to form a great many covalently bonded compounds in which there are chains of carbon atoms linked by single covalent bonds. In each case where the carbon atoms are joined to four other atoms the essential orientation around each carbon atom is tetrahedral. 

Methane, CH4, for example, has a central carbon atom bonded to four hydrogen atoms and the shape is a regular tetrahedron with a H—C—H bond angle of 109°28, exactly that calculated. Electrons in a lone pair , a pair of electrons not used in bonding, occupy a larger fraction of space adjacent to their parent atom since they are under the influence of one nucleus, unlike bonding pairs of electrons which are under the influence of two nuclei. Thus, whenever a lone pair is present some distortion of the essential shape occurs. 

In this case we have three bonding pairs and one lone pair. The essential shape is, therefore, tetrahedral but this is distorted due to the presence of the lone pair of electrons, the H—N—H bond angle beine 107 ... 

When the ammonium ion NH is formed the lone pair becomes a bonding pair and the shape becomes a regular tetrahedron. 

The distortion due to the presence of lone pairs of electrons is more marked in water ... 

If the spatial arrangement of atoms is required this can be deduced from the basic structure by neglecting the positions occupied by lone pairs of electrons. Water, for example, can be described as a V shape whilst ammonia is a trigonal pyramid. 

Other compounds containing lone pairs of electrons readily form co-ordinate links and in each case a change in spatial configuration accompanies the bond formation. The oxygen atom in dimethyl ether, CHj—O—CHj, has two lone pairs of electrons and is able to donate one pair to, for example, boron trichloride ... 

The ability to act as a lone pair acceptor is not confined to Group III, and can occur wherever a quantum level is incomplete. This ability to accept electrons explains why covalent chlorides, with the exception of carbon tetrachloride, are readily hydrolysed, the apparently anomalous behaviour of carbon tetrachloride being readily explained by the fact that the carbon has a completed quantum level and is unable to form an intermediate complex with water. 

The element before carbon in Period 2, boron, has one electron less than carbon, and forms many covalent compounds of type BX3 where X is a monovalent atom or group. In these, the boron uses three sp hybrid orbitals to form three trigonal planar bonds, like carbon in ethene, but the unhybridised 2p orbital is vacant, i.e. it contains no electrons. In the nitrogen atom (one more electron than carbon) one orbital must contain two electrons—the lone pair hence sp hybridisation will give four tetrahedral orbitals, one containing this lone pair. Oxygen similarly hybridised will have two orbitals occupied by lone pairs, and fluorine, three. Hence the hydrides of the elements from carbon to fluorine have the structures... 

The elements of Period 2 (Li—F) cannot have a co valency greater than 4, because not more than four orbitals are available for bonding. In Period 3 (Na—Cl) similar behaviour would be expected, and indeed the molecule SiH4 is tetrahedral like that of CH4, and PH3 is like NH3 with a lone pair occupying one tetrahedral position. 

What effect does the presence of a lone pair of electrons on the nitrogen atom have on ... 

As an example of a really strong base, the hydride ion H (for example in NaH) is unique it has one lone pair, a negative charge and a very small size. Like O , it is too strong a base to exist in water ... 

We have seen that a base can be defined as combining with a proton and, therefore, requires at least one lone pair of electrons. A more general definition of acids and bases, due to G. N. Lewis, describes a base as any species (atom, ion or molecule) which can donate an electron pair, and an acid as any species which can accept an electron pair— more simply, a base is an electron-pair donor, an acid an electron-pair acceptor. Some examples of Lewis acids and bases are ... 

Boron trioxide is not particularly soluble in water but it slowly dissolves to form both dioxo(HB02)(meta) and trioxo(H3B03) (ortho) boric acids. It is a dimorphous oxide and exists as either a glassy or a crystalline solid. Boron trioxide is an acidic oxide and combines with metal oxides and hydroxides to form borates, some of which have characteristic colours—a fact utilised in analysis as the "borax bead test , cf alumina p. 150. Boric acid. H3BO3. properly called trioxoboric acid, may be prepared by adding excess hydrochloric or sulphuric acid to a hot saturated solution of borax, sodium heptaoxotetraborate, Na2B407, when the only moderately soluble boric acid separates as white flaky crystals on cooling. Boric acid is a very weak monobasic acid it is, in fact, a Lewis acid since its acidity is due to an initial acceptance of a lone pair of electrons from water rather than direct proton donation as in the case of Lowry-Bronsted acids, i.e. 

This structure indicates that carbon monoxide should have donor properties, the carbon atom having a lone pair of electrons. Carbon... 

Ammonia is a colourless gas at room temperature and atmospheric pressure with a characteristic pungent smell. It is easily liquefied either by cooling (b.p. 240 K) or under a pressure of 8-9 atmospheres at ordinary temperature. Some of its physical and many of its chemical properties are best understood in terms of its structure. Like the other group head elements, nitrogen has no d orbitals available for bond formation and it is limited to a maximum of four single bonds. Ammonia has a basic tetrahedral arrangement with a lone pair occupying one position ... 

Ammonia as a donor molecule. Because of the presence of the lone pair of electrons on the nitrogen atom, ammonia can behave as an electron pair donor. For example, ammonia abstracts a proton from a water molecule producing the tetrahedral ammonium, NH4, ion and forms the compounds HjN- AlClj and HjN- BClj. 

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