Octet rule expanded

Tlie OCOO structure violates the octet rule (expanded octet). Tlie structure shown below satisfies the octet... 

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. 

The structures with expanded octets have lower formal charges, (b) There is one Lewis structure that obeys the octet rule ... 

This polyatomic ion has six bonds around the central Si atom, an obvious exception to the octet rule, so the central atom needs an expanded valence shell. 

B) To form OF there would have to be six bonds (twelve electrons around the oxygen atom). This would violate the octet rule. O does not have an empty d sublevel into which it can form expanded octets. S has an empty 3d sublevel that it uses to form six bonds in SF. ... 

Because the additional electrons must be accommodated in valence orbitals, only nonmetal atoms in Period 3 or higher can expand their octets. These elements have empty ci-orbitals in the valence shell. Another factor—possibly the main factor—in determining whether more atoms than allowed by the octet rule can bond to a central atom is the size of that atom. A P atom is big enough for up to six Cl atoms to fit comfortably around it, and PC1S is a common laboratory chemical. An N atom, though, is too small, and NC15 is unknown. 

FIGURE 6.10 The octet rule occasionally fails for the shaded main-group elements. These elements, all of which are in the third row or lower, can use low-energy unfilled d orbitals to expand their valence shell beyond the normal octet. 

There are a few notable exceptions to the octet rule that you should be aware of molecules with odd numbers of electrons, incomplete octets, and expanded octets. 

The octet rule applies quite well to the first full row of the periodic table (Li through F), but beyond this it is generally applicable only to the non-transition elements, and even in many of these it cannot explain many of the bonding patterns that are observed. The principal difficulty is that a central atom that is bonded to more than four peripheral atoms must have more than eight electrons around it if each bond is assumed to consist of an electron pair. In these cases, we hedge the rule a bit, and euphemistically refer to the larger number of electrons as an expanded octet . 

If the central element in a molecule or polyatomic ion is in the third period or higher and does not obey the octet rule, it is apt to expand its valence shell beyond 8 electrons. The phosphorus atom in PF5 has 10 electrons aronnd it ... 

When it is impossible to draw a structure consistent with the octet rule, it is necessary to increase the number of electrons around the central atom. An option limited to elements of the third and higher periods is to use d orbitals for this expansion, although more recent theoretical work suggests that expansion beyond the i and p orbitals is unnecessary for most main group molecules. In most cases, two or four added electrons will complete the bonding, but more can be added if necessary. Ten electrons are required around chlorine in CIF3 and 12 around sulfur in SFg (Figure 3-2). The increased number of electrons is described as an expanded shell or an expanded electron count. 

When an atom has a share of more than eight electrons, as does P in PF5, we say that it exhibits an expanded valence shell. The electronic basis of the octet rule is that one r and three p orbitals in the valence shell of an atom can accommodate a maximum of eight electrons. The valence shell of phosphorus has = 3, so it also has hd orbitals available that can be involved in bonding. It is for this reason that phosphorus (and many other representative elements of Period 3 and beyond) can exhibit an expanded valence shell. By contrast, elements in the second row of the periodic table can never exceed eight electrons in their valence shells, because each atom has only one s and three p orbitals in that shell. Thus, we understand why PF5 can exist but NF5 cannot. 

Lewis Covalent and Ionic Bonds - Lewis Structures - Octet Rule -Cations and Anions - Lone Pairs - Incomplete Octets - Expanded Octets - Double and Triple Bonds - Oxyacids - Resonance. 

This Structure involves an expanded octet on S but may be considered more plausible because it bears fewer formal charges. However, detailed theoretical calculation shows that the most likely structure is the one that satisfies the octet rule, even though it has greater formal charge separations. The general rule for elements in the third period and beyond is that a resonance structure that obeys the octet rule is preferred over one that involves an expanded octet but bears fewer formal charges. 

Give an example of an ion or molecule containing A1 that (a) obeys the octet rule, (b) has an expanded octet, and (c) has an incomplete octet. 

The six S—F bonds are formed by the overlap of the hybrid orbitals of the S atom and the 2p orbitals of the F atoms. Since there are 12 electrons around the S atom, the octet rule is violated. The use of d orbitals in addition to s and p orbitals to form an expanded octet (see Section 9.9) is an example of valence-shell expansion. Second-period elements, unlike third-period elements, do not have 2d energy levels, so they can never expand their valence shells. Hence atoms of second-period elements can never be surrounded by more than eight electrons in any of their compounds. 

We have already noted the exception of He, but for n> 3, there is the possibility of apparently expanding the octet (see p. 104). Although the octet rule is still useful at an elementary level, we must bear in mind that it is restricted to a relatively small number of elements. Its extension to the 18-electron rule, which takes into account the filling of ns, np and nd sub-levels, is discussed in Sections 20.4 and 23.3. 

Sulfur hexafluoride (4.5) provides an example of a so-called hypervalent molecule, i.e. one in which the central atom appears to expand its octet of valence electrons. However, a valence bond picture of the bonding in SFg involving resonance structures such as 4.6 shows that the S atom obeys the octet rule. A set of resonance structures is needed to rationalize the observed equivalence of the six... 

The octet rule is a useful guide for most molecules with Period 2 central atoms, but not for every one. Also, many molecules have central atoms from higher periods. As you ll see, some central atoms have fewer than eight electrons around them, and others have more. The most significant octet rule exceptions are for molecules containing electron-deficient atoms, odd-electron atoms, and especially atoms with expanded valence shells. 

He and H cannot serve as central atoms in a Lewis structure. Both can have no more than two valence electrons. Fluorine needs only one electron to complete its valence level, and it does not have d orbitals available to expand its valence level. Thus, it can bond to only one other atom. 10.3 All the structures obey the octet rule except c and g. 

Figure 8.17 Prior to the reaction of PCI3 and CI2, every reactant atom follows the octet rule. After the reaction, the product, PCI5, has an expanded octet containing ten electrons. 

Summarize exceptions to the octet rule by correctly pairing these molecules and phrases odd number of valence electrons, PCI5, CIO2, BFI3, expanded octet, less than an octet. 

The mosl spectacular variations from the octet rule are in xenon chemistry, where the rule would predict zero valency. Xenon forms a small range of compounds wii oxygen and fluorine in which il expands its valence shell to produce oxidalion states of +2,... 

In 1937 English chemist Nevil V. Sidgwick suggested a rule (the octet rule for first-row p-block elements) for complex formation tmder which a metal can acquire ligands until the total number of electrons around it is equal to the number surrounding the next noble gas. This rule was later expanded as the eighteen-electron rule under which a d-block transition metal atom has eighteen electrons in its nine valence orbitals [five n d one (n -I-... 

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