Which, as you can see, is just the two Lewis structures you would have normally drawn, except with a double-sided arrow between them.
The two diagrams shown are referred to as resonance structures. For example, if you look at only one of the Lewis structures individually, you may believe that only one of the oxygen atoms the one with the negative charge will be reactive and the other one will just sit around.
By looking at both of the Lewis structures, you can see that both of the oxygen atoms will be equally reactive because they both have some of the negative charge. How to draw resonance structures. Yep, all the atoms are combined and no rules have yet been broken.
Unfortunately, we have one more bond to add, and it pretty much has to be an N-O bond. But which oxygen will double bond to the nitrogen atom? I could put the bond between nitrogen and the oxygen atom on the left, and everything would be cool. I could put the bond between nitrogen and the oxygen atom on the right, and everything would also be cool. Which one do I do? If you have this kind of choice, the best thing to do is to draw two structures: One in which you follow one choice and one in which you follow the other.
When will I need resonance structures? If you are drawing a Lewis structure and find that you can put a multiple bond in two or more different locations, the resonance structures will correspond to each of these possible answers. When drawing resonance structures, this is the way in which you indicate the equivalence of all three resonance structures.
The images in this tutorial are all in the public domain, as are all scientific structures and formulas. The resources on this site were written between and by Ian Guch and are copyrighted. You must be logged in to post a comment. The Cavalcade o' Chemistry.
Celebrating 20 years of chemistry goodness. Seriously, we've been around for 20 years! Skip to content. Resonance structures Posted on September 18, by misterguch. As a result, we draw the nitrite ion in the following way: Which, as you can see, is just the two Lewis structures you would have normally drawn, except with a double-sided arrow between them.
Whereas the nitrate and nitrite ions have resonance structures as shown above , the hydroxide ion does not. In the case or aromatic or anti-aromatic and conjugated hydrocarbons, you may also see resonance structures. Again, not all organic compounds will have resonance structures, so only draw them if you have some compelling reason to do so. When we draw resonance structures, we convert lone pairs to bonds and bonds to lone pairs if it is possible.
In lewis structure NO 2 - ion, there are three lone pairs in the last shell in one oxygen atom and that oxygen atom is joint with nitrogen atom by a single bond. Also, that oxygen atom has a -1 charge. There is another oxygen atom. That oxygen atom is connected to the nitrogen atom by a double bond has two lone pairs in its last shell. Also, there is no charge in that oxygen atom. You can convert a lone pair of one oxygen atom which already has three lone pairs to make a bond with nitrogen atom.
With that, total electrons around nitrogen atom is going to be ten. It is going to break octal rule because nitrogen atom cannot keep more than eight electrons in its last shell. Then, what will do? Now, we convert a bond in the double bond between nitrogen atom and other oxygen atom to a lone pair on oxygen atom. Questions asked by students. Ask your question and find the answer free. Another way of saying this is that formal charge results when we take the number of valence electrons of a neutral atom, subtract the nonbonding electrons, and then subtract the number of bonds connected to that atom in the Lewis structure.
We can double-check formal charge calculations by determining the sum of the formal charges for the whole structure. The sum of the formal charges of all atoms in a molecule must be zero; the sum of the formal charges in an ion should equal the charge of the ion.
We must remember that the formal charge calculated for an atom is not the actual charge of the atom in the molecule. Formal charge is only a useful bookkeeping procedure; it does not indicate the presence of actual charges. Check Your Learning Calculate the formal charge for each atom in the carbon monoxide molecule:.
All atoms in BrCl 3 have a formal charge of zero, and the sum of the formal charges totals zero, as it must in a neutral molecule. The arrangement of atoms in a molecule or ion is called its molecular structure. In many cases, following the steps for writing Lewis structures may lead to more than one possible molecular structure—different multiple bond and lone-pair electron placements or different arrangements of atoms, for instance.
A few guidelines involving formal charge can be helpful in deciding which of the possible structures is most likely for a particular molecule or ion:.
To see how these guidelines apply, let us consider some possible structures for carbon dioxide, CO 2. We know from our previous discussion that the less electronegative atom typically occupies the central position, but formal charges allow us to understand why this occurs. We can draw three possibilities for the structure: carbon in the center and double bonds, carbon in the center with a single and triple bond, and oxygen in the center with double bonds:.
Comparing the three formal charges, we can definitively identify the structure on the left as preferable because it has only formal charges of zero Guideline 1. As another example, the thiocyanate ion, an ion formed from a carbon atom, a nitrogen atom, and a sulfur atom, could have three different molecular structures: CNS — , NCS — , or CSN —.
The formal charges present in each of these molecular structures can help us pick the most likely arrangement of atoms. Possible Lewis structures and the formal charges for each of the three possible structures for the thiocyanate ion are shown here:. Note that the sum of the formal charges in each case is equal to the charge of the ion —1.
However, the first arrangement of atoms is preferred because it has the lowest number of atoms with nonzero formal charges Guideline 2. Also, it places the least electronegative atom in the center, and the negative charge on the more electronegative element Guideline 4. Using Formal Charge to Determine Molecular Structure Nitrous oxide, N 2 O, commonly known as laughing gas, is used as an anesthetic in minor surgeries, such as the routine extraction of wisdom teeth.
Which is the likely structure for nitrous oxide? Solution Determining formal charge yields the following:. The structure with a terminal oxygen atom best satisfies the criteria for the most stable distribution of formal charge:.
The number of atoms with formal charges are minimized Guideline 2 , and there is no formal charge larger than one Guideline 2. This is again consistent with the preference for having the less electronegative atom in the central position. You may have noticed that the nitrite anion in Example 3 can have two possible structures with the atoms in the same positions.
The electrons involved in the N—O double bond, however, are in different positions:. If nitrite ions do indeed contain a single and a double bond, we would expect for the two bond lengths to be different. A double bond between two atoms is shorter and stronger than a single bond between the same two atoms. Instead, we use the concept of resonance : if two or more Lewis structures with the same arrangement of atoms can be written for a molecule or ion, the actual distribution of electrons is an average of that shown by the various Lewis structures.
0コメント