Electron-Dot Structures & Resonance:

The first way to envision a chemical bond is as an electron pair. Electron pairing is a natural phenomenon. Orbitals (atomic or molecular) are entities which can contain two paired electrons. Since the main part of many MO's happens to be centered in a bond, this idea, which is natural, is also correct--at least at the start. Where electrons are "delocalized" over more than just the volume we can give to a bond, extensions ofthe basic model--resonance structures--come into the model.

With the octet rule, things are simple:

Each atom gets 8 electrons (except H which gets two).

Each atom brings a number of valence electrons to a molecule equal to its group designation (e.g., C in 4A brings 4, O in 6A brings 6).
If a species is an ion, subtract one electron for each unit of positive charge; if an anion, add one electron for each unit of positive charge.

Each pair of electrons between atoms constitutes a bond. (Each bond counts as one electron; there are two shared).

Single, double, and triple bonds are possible.

Electrons belonging wholly to one atom are an electron pair (lone pairs belong wholly to the atom they are on).
For elements in the third row and higher in atomic number, the octet rule does not necessarily hold. You can determine whether or not by simply counting electrons and seeing how they fall into pairs.

The only way to really understand this is to actually do it! So, let's get going.

Problem 7.46: What is the octet rule and why does it apply primarily to main-group elements and not to transition metals?

Some of this was discussed just above in purple prose. We reiterate the most basic parts of the definition here and, possibly, look at things a little differently.

The octet rule states that main-grop elements tend to react so that they attain a noble gas configuration with filled s- and p-sublevels (8 electrons total, hence an octet).

The rule does break down in some places with representative elements. Most notable are the cases we discussed in problem 6.78 (elements in groups 3A-8A in the third row of the periodic table and lower).

Also quite notable occasional exceptions to the octet rule are the elements Be and B. Be should be ionic, one would think; however the "ion" is so small, that bonds are forced to be covalent. Thus means that the dot structure for a compound such as BeCl₂ comes out to be Cl:Be:Cl. Boron is even stranger.

BF₃ has just three electron pairs on the boron. This is partly due to the fact that F does not form double bonds and also to the fact that B is sometimes termed "electron deficient." It is too small to lose three electrons and attain [He]. And, asking it to take on 5 additional electrons vs. F is asking too much. However, species such as BF₄^-do form (in this case, a fluoride ion, being extremely rich in electrons, is more than glad to cooperate).

With hydrogen, boron forms numerous compounds, some very strange. For instance, borane, with the nominal formula BH₃, would be expected to have the same dot structure as BF₃. The is not the case, however. The correct formula is actually B₂H₆ and the bonding is rather complicated.

But, with elements in groups 4A, 5A, 6A, and 7A, the octet rule works more often than not and very rarely fails for C, N, O, or F!
We no proceed to some actual drawings.

Problem 7.48: Draw electron dot structures for the following molecules or ions:

These essentially follow the rules. The atoms where the octet rule is most important are in the second row of the periodic table. Note the presence of BF₄⁺ here (here B does get an octet).

(a)

CBr₄

32 valence electrons:

4 from C
28 from Br's

(b)

NCl₃

26 valence electrons:

5 from N
21 from Cl's

(c)

C₂H₅Cl

20 valence electrons:

5 from H's
8 from C's
7 from Cl

(d)

BF₄^-

32 valence electrons:

3 from B
28 from F's
1 added electron

(e)

O₂^2-

14 valence electrons:

12 from O's
2 from added electrons

(f)

NO⁺

10 valence electrons:

5 from N
6 from O
1 lost to positive charge

(The rightmost column was not asked for. It is included to aid you in your electron count. In the next example, it is assumed that you can add group numbers and add electrons for anions and subtract them if a cation!)

Problem 7.49: Draw electron dot structures for the following molecules which contain atoms from the third row or lower.

All these are a little bit tricky. Just remember to count valence electrons properly.

(a)	SbCl₃
(b)	KrF₂
(c)	ClO₂
(d)	PF₅
(e)	H₃PO₄
(f)	SeOCl₂

Problem 7.50: Draw as many resonance structures as you can for each of the following molecules or ions:

A rule we shall adhere to here is that the resonance structures be valid. (In problem 7.54 we shall see that the authors run into a roadblock.) Sorry these are so fat, but I had to get them all in at a decent size (and a little scrolling never hurt anyone). All resonance structures obey the octet rule (except for those where there is an odd number of electrons).

(a)	HN₃
(b)	SO₃
(c)	SCN^-

Problem 7.51: Draw as many resonance structures as you can for the following nitrogen-containing compounds:

No particular problems here if you are not confused occasionally by an odd number of electrons or by "fat" structures. Sorry about the latter, but it's fun infuriating those who waste time printing these web pages.

(a)	N₂O
(b)	NO
(c)	NO₂
(d)	N₂O₃ (ONNO₃)

Problem 7.52: Oxalic acid, H₂C₂O₄, is a poisonous substance found in uncooked spinach leaves. If oxalic acid has a C-C single bond and no C-H bond, draw its electron-dot structure.

Fairly easy. Incidentally, oxalic acid is the reason your teeth feel a little cleaner after eating spinach. The acid is not toxic enough to use as an excuse for not eating your spinach...

Problem 7.54: Which of the following pairs of structures represent resonance forms and which do not?

The authors say that all these are resonance structures. That may be true but some are NOT valid Lewis structures. So, we'll concede their first point and content ourselves here with saying which are valid and which are not.

The left structure is a valid structure. The one on the right is not since carbon has only 6 electrons. These are valence structures according to the authors but it should be noted that the one on the right is NOT a valid Lewis structure.

The structure on the left is valid; the one on the right is invalid since S has access to only 6 electrons.

Both these are valid Lewis structures. And they are also resonance structures.

These are definitely valid! This is the famous pair of structures for benzene!

Problem 7.56: Identify of third-row elements, X, that form the following ions:

We see 32 valence electrons. 28 come from the chlorines and 1 is added to make an anion. Also, "X" has to have donated 3 valence electrons. This means it is the 3A element, Al.

This cation has 8 valence electrons. 4 come from the H's and 5 would have to come from X since an electron was removed. The only element in the third row that could do this is phosphorus, P ("X" = "P").

Problem 7.59: Write electron-dot structures for molecules with the following connections:

With things like this, it is just a matter of getting the right number of electrons while following the octet rule. These should be easy (in hindsight...).

(I guess this was easy...).