Molecules Lewis structure

Elements in groups 2A and 3A of the periodic table, such as beryllium and boron, do not have enough valence electrons to form an octet in a neutral molecule. Lewis structures for BeH2 and BF3 show that these atoms have only four and six electrons, respectively, around the central atom. There is nothing we can do about this There simply aren t enough electrons to form an octet. 

Complete Table 8.9 by identifying the expected hybrid on the central atom. You might find drawing the molecules Lewis structure helpful. 

Molecule Lewis Structure Bond Bond Order Bond Energy (kJ/mole)... 

Molecule Lewis Structure Bond Bond Bond... 

You should be able to draw chemical formulas that show all of the valence electrons in a molecule (Lewis structures), using lines for bonds and dots to show unshared electrons. You should be proficient in representing structures as dash structural formulas, condensed structural formulas, and bond-line structural formulas. In particular, the more quickly you become skilled at using and interpreting bondline formulas, the faster you will be able to process structural information in organic chemistry. You have also learned about resonance structures, the use of which wiU help us in understanding a variety of concepts in later chapters. 

Molecule Lewis structure Number of electron pairs Electron pair arrangement Molecular shape... 

Molecule Lewis structure Sharing pattern Lewis structure... 

The original VSEPR theory (Figure 4.56) is a classical theory that accounts for the shapes of most main group molecules. Lewis structures are simple representations of molecules and ions that emphasize the importance of electron pairs (represented by dots and/or crosses). VSEPR theory proposes that the favoured molecular shape is that which minimizes the repulsion between electron pairs in the valence shell of the central atom. 

If it is assumed that an oxygen lone-pair electron delocalizes into a sulphur 3d orbital for either molecule, Lewis structures (11) and (12) are obtained. The latter... 

Lewis structures frequently contain atoms that bear a positive or negative charge If the molecule as a whole is neutral the sum of its positive charges must equal the sum of Its negative charges An example is nitric acid HNO3... 

Table 1 4 summarizes the procedure we have developed for writing Lewis structures Notice that the process depends on knowing not only the molecular formula but also the order m which the atoms are attached to one another This order of attachment is called the constitution, or connectivity, of the molecule and is determined by experiment Only rarely is it possible to deduce the constitution of a molecule from its molecular formula... 

When writing a Lewis structure we restrict a molecule s electrons to certain well defined locations either linking two atoms by a covalent bond or as unshared electrons on a sm gle atom Sometimes more than one Lewis structure can be written for a molecule espe cially those that contain multiple bonds An example often cited m introductory chem istry courses is ozone (O3) Ozone occurs naturally m large quantities m the upper atmosphere where it screens the surface of the earth from much of the sun s ultraviolet rays Were it not for this ozone layer most forms of surface life on earth would be dam aged or even destroyed by the rays of the sun The following Lewis structure for ozone satisfies fhe ocfef rule all fhree oxygens have eighf elecfrons m fheir valence shell... 

It IS good chemical practice to represent molecules by their most stable Lewis structure The ability to write alternative resonance forms and to compare their relative stabilities however can provide insight into both molecular structure and chemical behavior This will become particularly apparent m the last two thirds of this text where the resonance concept will be used regularly... 

Electron delocalization stabilizes a molecule A molecule in which electrons are delocalized is more stable than implied by any of the individual Lewis structures that may be written for it The de gree of stabilization is greatest when the contrib uting Lewis structures are of equal stability... 

The opening paragraph of this chapter emphasized that the connection between structure and properties is what chemistry is all about We have just seen one such con nection From the Lewis structure of a molecule we can use electronegativity to tell us about the polarity of bonds and combine that with VSEPR to predict whether the mol ecule has a dipole moment In the next several sections we 11 see a connection between structure and chemical reactivity as we review acids and bases... 

Table 14m this section sets forth the procedure to be followed m writ mg Lewis structures for organic molecules It begins with experimentally determined information the molecular formula and the constitution (order m which the atoms are connected)... 

Many molecules can be represented by two or more Lewis structures that differ only m the placement of electrons In such cases the electrons... 

Wnte a Lewis structure for each of the following organic molecules... 

The atoms in rnethylketene (C3H4O) are connected in the order and according to the geom etry shown (You can view this model in more detail on Learning By Modeling) Determine the hybridization state of each carbon and write a Lewis structure for this neutral molecule... 

Ozone (O3) IS the triatomic form of oxygen It is a neutral but polar molecule that can be represented as a hybrid of its two most stable Lewis structures... 

Although Lewis structures of this type are not entirely adequate descriptions of the structure of the excited states, they do correspond to the MO picture by indicating distortion of chaige and the presence of polar or radical-like centers. The excited states are much more reactive than the corresponding ground-state molecules. In addition to the increased energy content, this high reactivity is associated with the presence of half-filled orbitals. The two SOMO orbitals in the excited states have enhanced radical, cationic, or anionic character. 

To deal with circumstances such as the bonding in ozone, the notion of resonance between Lewis structures was developed. According to the resonance concept, when more than one Lewis structure may be written for a molecule, a single structure is insufficient to describe it. Rather, the true structure has an electron distribution that is a hybrid of all the possible Lewis structures that can be written for the molecule. In the case of ozone, two equivalent Lewis structures may be written. We use a double-headed aiTow to represent resonance between these two Lewis structures. 

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