 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.

EXAMPLE

Draw an acceptable Lewis structure for the nitrite ion, NO21-.

1. The N atom will be central in the system because it is the more electropositive element (also, since there is one nitrogen atom and two oxygen atoms, the unique atom rule would suggest nitrogen as the central atom).

O N O

2. The total number of valence electrons is 18. Six electrons are contributed from each of two oxygen atoms and five electrons from the nitrogen. Since the species is a monoanion, it has one electron besides those provided by the atoms.

6 + 6 + 5 + 1 = 18 valence electrons

3. After making electron pair bonds between the N and each O atom, 14 valence electrons remain to place in the ion.

O - N - O

18 - 4(i.e., 2 for each bond) = 14 valence electrons remain

4. After completing the octets of the oxygen atoms (by placing three lone pairs on each), two valence electrons remain. 14 - 12(i.e., 6 to each oxygen) = 2

5. The final two electrons are placed, as a pair, on the central nitrogen atom. 6. Because the nitrogen atom has only six electrons surrounding it, a double bond must be formed. By taking a lone pair from either oxygen and making it a shared pair between that oxygen and the nitrogen all atoms achieve an octet of electrons. Notice that making this double bond does not affect the electron count around the oxygen but increases by two the electrons around nitrogen. In order to indicate the ionic charge, the drawing is enclosed in brackets and the charge is shown at the upper right. 7. Since the double bond could have instead been formed between the nitrogen and the other oxygen, resonance is important. The actual form of the ion is a hybrid of these two Lewis structures. There are a few exceptions to the octet rule.(3) 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.

a) I31-

b) XeO4

c) ClF3

d) CO32- Click Here For Answers

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.