Writing Lewis Structures

Lewis structure Writing of formulas by representing Pauli s Principle When two electrons occupy the same valence electrons by dots. orbital, they must have opposite spins. 

For each of the formulas listed below, draw a reasonable Lewis structure, write the name of the electron group geometry around the central atom, draw the geometric sketch of the molecule, including bond angles, and write the name of the molecular geometry around the central atom. 

For three simple molecules of your own choice, apply the rules for writing Lewis structures. Write your discussion as if you are explaining the method to someone who is not familiar with Lewis structures. 

EXAMPLE 10.2 Writing Ionic Lewis Structures Write the Lewis structure of the compound MgO. ... 

Free radicals are molecules that contain an odd number of valence electrons and therefore contain an unpaired electron in their Lewis structure. Write the best possible Lewis structure for the free radical FIOO. Does Lewis theory predict that HOO is stable Predict its geometry. 

Write the most stable Lewis structure for each of the following... 

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... 

Keeping the same atomic connections and moving only electrons write a more stable Lewis structure for each of the following Be sure to specify formal charges if any in the new structure... 

Write the Lewis structure and include unshared electron pairs... 

The Lewis rules are relatively straightforward easiest to master and the most familiar You will find that your ability to write Lewis formulas increases rapidly with experience Get as much practice as you can early m the course Success m organic chemistry depends on writing correct Lewis structures... 

Molecular models such as the one shown often do not explicitly show double and triple bonds Write a Lewis structure for this hydrocarbon showing the location of any multiple bonds Specify the hybndization state of each carbon (You can view this model in more detail on Learn mg By Modeling)... 

Table 1.4 in this section sets forth the procedure to be followed in writing Lewis structures for organic molecules. It begins with experimentally deter-mined infor-mation the molecular formula and the constitution (order in which the atoms are connected). 

All the following compounds are characterized by ionic bonding between a group I metal cation and a tetrahedral anion. Write an appropriate Lewis structure for each anion, remembering to specify formal charges where they exist. 

When we say cycloheptatriene is not aromatic but cycloheptatrienyl cation is, we are not comparing the stability of the two to each other. Cycloheptatriene is a stable hydrocarbon but does not possess the special stability required to be called aromatic. Cycloheptatrienyl cation, although aromatic, is still a carbocation and reasonably reactive toward nucleophiles. Its special stability does not imply a rock-like passivity, but rather a much greater ease of formation than expected on the basis of the Lewis structure drawn for it. A number of observations indicate that cycloheptatrienyl cation is far-more stable than most other car bocations. To emphasize its aromatic nature, chemists often write the structure of cycloheptatrienyl cation in the Robinson circle-in-a-ring style. 

Examine the geometry and atomic charges of 2-pyridone to see if it is localized as indicated in the drawing above, or delocalized (as in 2-hydroxypyridine). If you need to, write alternative Lewis structures to that provided above. How many n electrons does 2-pyridone possess Is 2-pyridone aromatic ... 

These structures (without the circles) are referred to as Lewis structures. In writing Lewis structures, only the valence electrons written above are shown, because they are the ones that participate in covalent bonding. For the main-group elements, the only ones dealt with here, the number of valence electrons is equal to the last digit of the group number in the periodic table (Table 7.1). Notice that elements in a given main group all have the same number of valence electrons. This explains why such elements behave similarly when they react to form covalently bonded species. 

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