Lewis Structures for Polyatomic Species

We begin to write Lewis structures for polyatomic species just as we did for methane ... 

Sometimes when writing the Lewis structure of a species, we may draw more than one possible correct Lewis structure for a molecule. The nitrate ion, N03 , is a good example. The structures that we write for this polyatomic anion differ in which oxygen has a double bond to the nitrogen. None of these three truly represents the actual structure of the nitrate ion—it is an average of all three of these Lewis structures. We use resonance theory to describe this situation. Resonance occurs when more than one Lewis structure (without moving atoms) is possible for a molecule. The individual structures are called resonance structures (or forms) and are written with a two-headed arrow (<- ) between them. The three resonance forms of the nitrate ion are ... 

Lewis structures can tell a lot about the bonding in covalent species, and they can be derived for almost all small molecules and polyatomic ions by following a prescribed sequence of operations. Let s go through this sequence and derive the Lewis structures for both the nitrate ion, N03 , and the formaldehyde molecule, H2CO, at the same time. 

After the Lewis structure for a molecule or polyatomic ion is developed, the shape of the species can be determined using the information in this table as a guide. 

To write a Lewis structure, we decide on the relative placement of the atoms in the molecule (or polyatomic ion)—that is, which atoms are adjacent and become bonded to each other—and distribute the total number of valence electrons as bonding and lone pairs. Let s begin by examining Lewis structures for species that obey the octet rule—those in which each atom fills its outer level with eight electrons (or two for hydrogen). 

Lewis structures (Lewis formulas or electron-dot formulas) are two-dimensional pictures of covalent species that show how the atoms are joined together with covalent bonds. A bond is shown as a pair of dots (2 dots = 2 electrons) or a dash (-), which represents a bonding or shared pair of electrons. A single dash represents two shared electrons two dashes (=) represent four shared electrons and so forth. In addition, Lewis structures also show the location of electron pairs not used in bonds, the nonbonding or unshared pairs of electrons. In a correct Lewis structure, all the valence electrons from every atom in the molecule or polyatomic ion must be accounted for, either in bonds or as nonbonding pairs (nb-pairs). 

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