Lewis structures validity

To check on the validity of a Lewis structure, verify that each atom has an octet or a duplet. As we shall see in Section 2.10, a common exception to this rule arises when the central atom is an atom of an element in Period 3 or higher. Such an atom can accommodate more than eight electrons in its valence shell. Consequently, the most stable Lewis structure may be one in which the central atom has more than eight electrons. 

In the solid state, sulfur is sometimes found in rings of six atoms, (a) Draw a valid Lewis structure for Sb. (b) Is resonance possible in S6 If so, draw one of the resonance structures. 

Lewis s theory also fails to account for the compound diborane, B2H6, a colorless gas that bursts into flame on contact with air. The problem is that diborane has only 12 valence electrons (three from each B atom, one from each H atom) but, for a Lewis structure, it needs at least seven bonds, and therefore 14 electrons, to bind the eight atoms together Diborane is an example of an electron-deficient compound, a compound with too few valence electrons to be assigned a valid Lewis structure. Valence-bond theory can account for the structures of electron-deficient compounds in terms of resonance, but the explanation is not straightforward. 

The boranes are electron-deficient compounds (Section 3.8) we cannot write valid Lewis structures for them, because too few electrons are available. For instance, there are 8 atoms in diborane, so we need at least 7 bonds however, there are only 12 valence electrons, and so we can form at most 6 electron-pair bonds. In molecular orbital theory, these electron pairs are regarded as delocalized over the entire molecule, and their bonding power is shared by several atoms. In diborane, for instance, a single electron pair is delocalized over a B—H—B unit. It binds all three atoms together with bond order of 4 for each of the B—H bridging bonds. The molecule has two such bridging three-center bonds (9). 

Let us first inquire whether basic criteria for the validity of low-order perturbation theory are actually satisfied in the present case. As described in Section 1.4, the perturbative starting point is an idealized natural Lewis-structure wavefunction (t//,l )) of doubly occupied NBOs. The accuracy of this Lewis-type starting point may be assessed in terms of the percentage accuracy of the variational energy (E) or density (p(l ). as shown for each molecule in Table 3.20. 

Figure 11.11 shows there are some molecules which can legitimately be drawn in several different ways using Lewis structures, each conforming to the octet rule. These are resonance structures, and are equally valid, but the true structure is a hybrid of the two or more possible structures. This is indicated by the double-headed arrow, where the electrons are moved, but the atoms stay in position. However, in this example, the carbon oxygen bonds are of equal length - they do not rapidly interconvert from one version to another. The true... 

All structures must be valid Lewis structures. An atom may become electron deficient, but, on the other hand, it must never be shown with more valence electrons than it can accommodate. For example, it is not possible to have pentavalent carbon. 

Note that a Lewis structure says nothing about the bond angles in the molecule since both (a) and (b) meet ad the criteria for a valid Lewis structure. 

A molecule or ion for which two or more valid Lewis structures can be drawn, differing only in the placement of the valence electrons. These Lewis structures are called resonance forms or resonance structures. Individual resonance forms do not exist, but we can estimate their relative energies. The more important (lower-energy) structures are called major contributors, and the less important (higher-energy) structures are called minor contributors. When a charge is spread over two or more atoms by resonance, it is said to be delocalized and the molecule is said to be resonance stabilized, (pp. 14-18)... 

Begin with a valid Lewis structure, and use hybrid orbitals for the sigma bonds and lone pairs. Use pi bonds between unhybridized p orbitals for the second and third bonds of double and triple bonds. 

All the resonance structures must be valid Lewis structures for the compound. 

There is a rather large class of molecules for which one has no difficulty writing Lewis structures in fact, we can write more than one valid structure for a given molecule. This raises a new problem how do we decide which one to use ... 

Resonance forms must be valid Lewis structures and obey normal rules of valency. 

Resonance occurs when one or more valid Lewis structures exist for a molecule or polyatomic ion. The structures that represent the substance are called resonance structures. Each resonance structure does not characterize the substance, but the average of all the resonance structures represents the molecule or polyatomic ion. Resonance structures are usually placed in brackets and separated by a double-headed arrow (<- ). 

Resonance Occurs when one or more valid Lewis structures exist for a molecule or polyatomic ion. Scientific method A framework for the stepwise process to experimentation. 

Resonance forms must he valid Lewis structures and obey normal rules of valency. A resonance form is like any other structure The octet rule still applies.. For example, one of the following structures for the acetate ion is noV a valid resoriance fViv because the carbon atom has five bonds and ten valence electrons ... 

A valid Lewis structure is one that obeys the rules we have outlined. 

Sometimes more than one valid Lewis structure is possible for a given molecule. For example, consider the Lewis structure for the nitrate ion (NOj-), which has 24 valence electrons. So that an octet of electrons surrounds each atom, a structure like the following is required ... 

Look again at the proposed Lewis structure for NO3-. Because there is no reason for choosing a particular oxygen atom to have the double bond, there are really three valid Lewis structures ... 

The nitrate ion does not exist as any of the three extreme forms indicated by the individual Lewis structures but instead exists as an average of all three. Resonance is invoked when more than one valid Lewis structure can be written for a particular molecule. The resulting electron structure of the molecule is given by the average of these resonance structures. This situation is usually represented by double-headed arrows as follows ... 

In this same vein, note the difference between a correct, or valid, Lewis structure and an electronic structure that accurately accounts for a molecule s observed properties. A valid Lewis structure is one that obeys the rules we have established for Lewis structures. However, this Lewis structure may or may not give an accurate picture of the molecule and its properties. Experiments must be carried out to make the final decisions on the correct description of the bonding in a molecule or polyatomic ion. ... 

In this section we will attempt to make these simple models even more useful by addressing a particular shortcoming of the LE model—its assumption that electrons are localized (restricted to the space between a given pair of atoms). This problem is most apparent for molecules where several valid Lewis structures can be drawn. Recall that none of the resonance structures taken alone adequately describes the electronic structure of the molecule. The concept of resonance was invented to solve this problem. However, even with... 

Adding four two-electron bonds around carbon uses all eight valence electrons, and so there are no lone pairs. To check whether a Lewis structure is valid, we must answer YES to three questions ... 

The answer to all three questions is YES, so the Lewis structure drawn for CH4 is valid. 

In Step [3], placing bonds between all atoms uses only 10 electrons, and the O atom does not yet have a complete octet. To complete the structure, give the O atom two nonbonded electron pairs. This uses all 14 electrons, giving every H two electrons and every second-row element eight. We have now drawn a valid Lewis structure. 

Probism 1.5 Draw a valid Lewis structure for each species a. CH3CH3 b. CH5N c. CHs ... 

This uses all 12 electrons, each C has an octet, and each H has two electrons. The Lewis structure is valid. Ethylene contains a carbon-carbon double bond. 

Both are valid Lewis structures, and both molecules exist. One is called ethanol, and the other, dimethyl ether. These two compounds are called isomers. 

Some molecules can t be adequately represented by a single Lewis structure. For example, two valid Lewis structures can be drawn for the anion (HCONH). One stmcture has a negatively charged N atom and a C - O double bond the other has a negatively charged O atom and a C - N double bond. These structures are called resonance structures or resonance forms. A doubleheaded arrow is used to separate two resonance structures. 

Rule [3] Resonance structures must be valid Lewis structures. Hydrogen must have two electrons and no second-row element can have more than eight electrons. 

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