Valence electrons octet rule

Carbocation has a positively charged carbon atom which has only six electrons in its outer valence shell instead of the eight valence electrons (octet rule). 

When atoms form covalent bonds, they try to attain a valence-electron configuration similar to that of the following noble gas element. When the elements in the first few horizontal rows of the periodic table form covalent bonds, they attempt to achieve the configurations of the noble gases helium (two valence electrons, duet rule) and neon and argon (eight valence electrons, octet rule). 

Figure 2-19. The BE-matriK of ethanal allows one to determine tine number of valence electrons (the sum of each row) on the atoms and to validate the octet rule,...

Lewis structures in which second row elements own or share more than eight valence electrons are especially unstable and make no contribution to the true structure (The octet rule may be ex ceeded for elements beyond the second row)... 

Section 1 3 The most common kind of bonding involving carbon is covalent bond ing A covalent bond is the sharing of a pair of electrons between two atoms Lewis structures are written on the basis of the octet rule, which limits second row elements to no more than eight electrons m their valence shells In most of its compounds carbon has four bonds... 

There is ample evidence from a variety of sources that carbocations are mterme diates m some chemical reactions but they are almost always too unstable to isolate The simplest reason for the instability of carbocations is that the positively charged car bon has only six electrons m its valence shell—the octet rule is not satisfied for the pos itively charged carbon... 

Octet rule (Section 1.3) When forming compounds, atoms gain, lose, or share electrons so that the number of their valence electrons is the same as that of the nearest noble gas. For the elements carbon, nitrogen, oxygen, and the halogens, this number is 8. 

Rule 4 Resonance forms obey norma] rules of valency. A resonance form is like any other structure the octet rule still applies to main-group atoms. For example, one of the following structures for the acetate ion is not a valid resonance form because the carbon atom has five bonds and ten valence electrons ... 

Although most of the molecules and polyatomic ions referred to in general chemistry follow the octet rule, there are some familiar species that do not. Among these are molecules containing an odd number of valence electrons. Nitric oxide, NO, and nitrogen dioxide, N02, fall in this category ... 

The largest class of molecules to violate the octet rule consists of species in which the central atom is surrounded by more than four pairs of valence electrons. Typical molecules of this type are phosphorus pentachloride, PC15, and sulfur hexafluoride, SF6. The Lewis structures of these molecules are... 

Lewis s interest in chemical bonding and structure dated from 1902. In attempting to explain "valence" to a class at Harvard, he devised an atomic model to rationalize the octet rule. His model was deficient in many respects for one thing, Lewis visualized cubic atoms with electrons located at the corners. Perhaps this explains why his ideas of atomic structure were not published until 1916. In that year, Lewis conceived of the... 

In 1923. Lewis published a classic book (later reprinted by Dover Publications) titled Valence and the Structure of Atoms and Molecules. Here, in Lewis s characteristically lucid style, we find many of the basic principles of covalent bonding discussed in this chapter. Included are electron-dot structures, the octet rule, and the concept of electronegativity. Here too is the Lewis definition of acids and bases (Chapter 15). That same year, Lewis published with Merle Randall a text called Thermodynamics and the Free Energy of Chemical Substances. Today, a revised edition of that text is still used in graduate courses in chemistry. 

A major weakness of valence bond theory has been its inability to predict the magnetic properties of molecules. We mentioned this problem in Chapter 7 with regard to the 02 molecule, which is paramagnetic, even though it has an even number (12) of valence electrons. The octet rule, or valence bond theory, would predict that all the electrons in 02 should be paired, which would make it diamagnetic. 

Octet rule The principle that bonded atoms (except H) tend to have a share in eight valence electrons, 166-171 exceptions to, 172-176 molecular geometry and, 181t molecular orbitals and, 650 Octyl acetate, 596t Open-pit copper mine, 540 Oppenheimer, J. Robert, 523 Optical isomer Isomer which rotates the... 

The octet rule accounts for the valences of many of the elements and the structures of many compounds. Carbon, nitrogen, oxygen, and fluorine obey the octet rule rigorously, provided there are enough electrons to go around. However, some compounds have an odd number of electrons. In addition, an atom of phosphorus, sulfur, chlorine, or another nonmetal in Period 3 and subsequent periods can accommodate more than eight electrons in its valence shell. The following two sections show how to recognize exceptions to the octet rule. 

The octet rule tells us that eight electrons fill the outer shell of an atom to give a noble-gas ns1ns(l valence-shell configuration. However, when the central atom in a molecule has empty d-orbitals, it may be able to accommodate 10, 12, or even more electrons. The electrons in such an expanded valence shell may be present as lone pairs or may be used by the central atom to form additional bonds. 

To understand Lewis ideas of acids and bases, it is necessary to understand a little bit about valence electrons and the octet rule. 

In Li2Sb we can assume Sb2- particles with seven valence electrons. Therefore, we expect Sb2 dumbbells (isoelectronic with I2) and observance of the octet rule. In fact, such dumbbells are present in the structure (Sb-Sb bond length 297 pm) however, this applies only to half of the Sb atoms. The other half form linear chains of Sb atoms (Sb-Sb distance 326 pm). For the bonds in the chain we assume a band according to Fig. 10.5 (p. 93) every Sb atom contributes to this band with one p orbital and one electron. With one electron per Sb atom the band is half-occupied, and therefore it is bonding. The... 

Lewis recognized that certain molecules such a PCI5 and SF6 are exceptions to the octet rule because their Lewis structures indicate that the central atom has more than eight electrons in its valence shell 10 for the P atom in PCI5 and the S atom in SF4, and 12 for the S atom in SFg (Figure 1.17). Such molecules are called hypervalent because the valence of the central atom is greater than its principal valence. To write a Lewis structure for such molecules, the Lewis symbol for the hypervalent atom must be modified to show the correct number of unpaired electrons. For the molecules in Figure 1.17 we would need to write the Lewis symbols as follows ... 

There are also molecules that are exceptions to the octet rule because one of the atoms has fewer, rather than more than, eight electrons in its valence shell in the Lewis structure (Figure 1.19). These molecules are formed by the elements on the left-hand side of the periodic table that have only one, two, or three electrons in their valence shells and cannot therefore attain an octet by using each of their electrons to form a covalent bond. The molecules LiF, BeCl2, BF3, and AIC13 would be examples. However, as we have seen and as we will discuss in detail in Chapters 8 and 9, these molecules are predominately ionic. In terms of a fully ionic model, each atom has a completed shell, and the anions obey the octet rule. Only if they are regarded as covalent can they be considered to be exceptions to the octet rule. Covalent descriptions of the bonding in BF3 and related molecules have therefore... 

Figure 1.19 Some examples of molecules that are exceptions to the octet rule because the central atom has fewer than eight electrons in its valence shell.

We should note that hydrogen never has more than two electrons in its valence shell in the Lewis diagram of any of its molecules because its valence shell is filled by just two electrons. Thus the octet rule is not applicable to hydrogen. 

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