Drawing Lewis Structures - A Review(1)
A Lewis structure is used to show the connectivity of atoms in a molecule (i.e., to indicate which atoms are bound together) and to represent how the valence electrons could be placed in the molecule to provide each atom with eight electrons.(2) A Lewis structure itself does not illustrate molecular shape, however, the structure can be used to deduce molecular geometry. It is generally possible to produce an acceptable Lewis structure for a molecule or polyatomic ion by working through several steps in order.
Sketch the atomic arrangement. This arrangement typically consists of one central atom
surrounded by the remaining atoms. Usually the central atom is the most electropositive
element in the compound (hydrogen, however, is never central). The "unique atom rule"
can also be helpful when identifying the central atom. This rule is based on the fact that,
for many compounds, there is a single atom of some element and multiple atoms of other
elements. This unique atom is central (e.g., the I in IO41-, S in H2SO4, C in CH2Br2). For
oxyacids, each acidic hydrogen is attached to an oxygen atom. The central atom then, is
the element other than oxygen or hydrogen.
Determine the total number of valence electrons. The units digit of the group number of an element gives the number of valence electrons that atom possesses. For instance F, in group 17, contains seven valence electrons.
Make electron pair bonds between the central atom and each outer atom.
Complete the octet of all outer atoms (except, of course hydrogen atoms).
Any remaining electrons are placed on the central atom.
If (and only if) the central atom still has fewer than eight electrons, it will be necessary to make multiple bonds. Complete the octet of the central atom by taking a lone pair of electrons from any outer atom and form a second shared pair bond between the outer atom and the central atom.
If resonance is important, draw all significant forms.
Draw an acceptable Lewis structure for the nitrite ion, NO21-.
18 - 4(i.e., 2 for each bond) = 14 valence electrons remain
14 - 12(i.e., 6 to each oxygen) = 2
|H atoms require exactly two electrons.|
|Be atoms require four electrons.|
|Group 13 atoms require six electrons.|
|Odd electron species cannot provide eight electrons to each atom. In such cases the central atom will be surrounded by seven electrons in the final Lewis structure. Recall that the central atom is more electropositive, and thus will logically be the electron deficient center.|
|Elements below period two can exceed eight electrons in the valence shell. Do not, however, exceed an octet of electrons if it can be avoided (i.e., do not form multiple bonds if doing so would place more than eight electrons around an atom).|
Exercise: Draw an acceptable Lewis structure for the following. If resonance is important, draw
all significant resonance forms.
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1. This review was prepared by Dr. David Rieck for use in courses offered by the Department of Chemistry at Salisbury University.
2. There are a number of exceptions to this "octet rule." A hydrogen atom for example, because it has only a 1s orbital in the valence shell, can accommodate only two electrons. In most simple cases, however, the "best" Lewis structure will place exactly eight valence electrons on each atom.
3. The list of exceptions provided is not exhaustive. In addition, many of the atoms which appear to be electron deficient "exceptions" are good Lewis acids, accepting a pair of electrons and thereby achieving a complete octet.