Lewis structures octet rule, exceptions

SAMPLE PROBLEM 10.5 Writing Lewis Structures for Octet-Rule Exceptions Problem Write Lewis structures for (a) H3PO4 (pick the most likely structure) (b) BFCI2. Plan We write each Lewis structure and examine it for exceptions to the octet rule. In (a), the central atom is P, which is in Period 3, so it can use d orbitals to have more than an octet. Therefore, we can write more than one Lewis structure. We use formal charges to decide if one resonance form is more important. In (b), the central atom is B, which can have fewer than an octet of electrons. 

These species do not obey the octet rule. Draw a Lewis structure for each, and state the type of octet-rule exception ... 

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. 

Knowledge Required (1) The rules for drawing Lewis structures. (2) The exceptions to the octet rule. 

SAMPLE PROBLEM 10.5 Writing Lewis Structures for Octet-Rule Exceptions... 

Reality Check Whenever you write a Lewis structure, check to see if it follows the octet rule. The structures written for OCl and C2H6 do just that each atom except H is surrounded by eight electrons. 

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. 

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... 

Not only molecules with LLPCN > 4, but all molecules of the elements in period 3 and beyond in their higher valence states, including most of their numerous oxides, oxoacids, and related molecules such as SO3 and (H0)2S04 should be regarded as hypervalent if AO bonds are described as double bonds (1). However, Lewis did not regard these molecules as exceptions to the octet rule because he wrote the Lewis structures of these molecules with single bonds and the appropriate formal charges (2). 

Even though we have an exception, we can still complete the Lewis structure. We need to draw a bond from each of the fluorine atoms to the central xenon. This gives us 4 bonds and uses 8 electrons. Each fluorine atom needs to complete its octet. The bond accounts for 2 electrons, so we need 6 more electrons (3 pairs) for each. Therefore, we add 3 separate pairs to each of the fluorine atoms. Six electrons per fluorine times 4 fluorine atoms accounts for 24 electrons. Our Lewis structure now contains 8 + 24 = 32 electrons. The number of available electrons (A) is 36, so we still need to add 36 - 32 = 4 electrons. These 4 electrons will give us 2 pairs. The xenon atom will get these pairs and become an exception to the octet rule. The actual placement of the pairs is not important as long as it is obvious that they are with the central atoms and not one of the fluorine atoms. The final Lewis structure is ... 

The Lewis dot structure for BF3 shows the central boron atom being surrounded by only six electrons, which violates the octet rule. This illustrates that the octet rule, while providing general guidelines, has exceptions. 

Draw a single bond (one pair of electron dots or a line) between each pair of connected atoms. Place the remaining electrons around the atoms as unshared pairs. If every atom has an octet of electrons except H, He, Li, and Be, which are atoms with two electrons, the Lewis structure is complete. Shared electrons count towards both atoms. If there are too few electron pairs to do this, draw multiple bonds (two or three pairs of electron dots between the atoms) until an octet is around each atom (except H atoms with two). If there are two many electron pairs to complete the octets with single bonds then the octet rule Is broken for this compound. 

Write Lewis structures that obey the octet rule for each of the following. Except for HCN and H2CO, the first atom listed is. the central atom. For HCN and H2CO, carbon is the central atom. 

The discussion of Lewis structures concludes with the introduction of isomers and exceptions to the octet rule. 

You may have to try several Lewis structures until you get one in which all of the atoms, except hydrogen, beryllium, and boron, obey the octet rule. 

The Lewis structure is focused on the attainment of an octet by all atoms when they bond with other elements. Some molecules and ions, however, do not obey the octet rule. Three reasons exist for these exceptions. 

The octet rule predicts that atoms form enough covalent bonds to surround themselves with eight elechons each. When one atom in a covalently bonded pair donates two electrons to the bond, the Lewis structure can include the formal charge on each atom as a means of keeping track of the valence electrons. There are exceptions to the octet rule, particularly for covalent beryllium compounds, elements in Group 3A, and elements in the third period and beyond in the periodic table. 

Blood Pressure and the Sodium lon/Potassium Ion Ratio Lewis Structures of Polyatomic Ions Lewis Structure, Stability, Multiple Bonds, and Bond Energies Lewis Structures and Resonance Lewis Structures and Exceptions to the Octet Rule... 

Drawing Lewis Structures of Covalently Bonded Compounds That Are Exceptions to the Octet Rule... 

Describe the three exceptions to the octet rule, draw Lewis structures for them, and use formal charges to select the most... 

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