Lewis Structures

Basic Method

• Arrange the atoms in the correct layout. This is either given or can be guessed. For a molecule of formula AX_n, A is usually the central atom with the X atoms all attached to it.
• Determine the number of electrons available from the group numbers of the elements.
• Draw an initial structure by
• Putting single bonds between neighboring atoms;
• Putting lone pairs on atoms to give each an octet (except H) until the number of electrons counted above has been place in the structure or until every atom has an octet.
• Alter the initial structure if necessary
• If the initial structure shows every atom with an octet, the structure is correct
• If one or more atoms in the initial structure have fewer than 8 electrons, use lone pairs to form multiple (double, triple) bonds to these atoms until every atom has 8 (except H)
• If every atom has an octet but not all available electrons have been used in the initial structure, add all remaining electrons to the central atom. In these cases, the central atom will be from period > 3.
Examples:
SO₂
 
• atom layout O-S-O
• Count electrons: 6(S) + 2(6)(O) = 18
• Draw initial structure:


• Alter initial structure if necessary

As drawn, the initial structure shows sulfur with only 6 electrons. Use a lone pair from the oxygen atom on the right to form a double bond to sulfur. This gives sulfur 8 electrons without changing the number of electrons on the oxygen atom or the total number of electrons in the structure (which must be 18). Note that we could just as well have used a lone pair from the left oxygen.



SO₄^2-
 

• atom layout


• Count electrons: 6(S) + 4(6)(oxygen) + 2 (double negative charge) = 32
• Draw initial structure


• Alter initial structure if necessary. The initial structure is fine as drawn, because it shows the correct total number of electrons, and every atom has an octet.
XeF₂
 
• atom layout F-Xe-F
• Count electrons: 8(Xe) + 2(7)(F) = 22
• Draw initial structure (see below)
• Alter initial structure if necessary.

As drawn, the initial structure shows every atom with an octet, but with only 20 electrons. The remaining 2 electrons are placed on Xe:

• Avoid large formal charges on atoms
• Avoid having FC of same sign on adjacent atoms.
• Avoid large separation of opposite charges.
• FCs should be consistent with atom electronegativities.

Resonance

When 2 or more reasonable Lewis structures can be drawn for a molecule, the actual structure is a composite of the contributing forms, called resonance forms. We indicate resonance using a double headed arrow

The actual structure is more stable than either of the (hypothetical) contributing forms.

When 2 resonance forms are equivalent, they contribute equally to the true structure. For example

When 2 forms are non-equivalent, they contribute in proportion to their acceptability.

Procedure for developing resonance forms, applied to H₂SO₄:

1. Decide on an arrangement for the atoms. Once arranged, the atoms may not be moved. For sulfuric acid, the arrangement is


2. Determine an acceptable Lewis structure, usually the one resulting directly from the procedure presented above. For sulfuric acid, this is


3. Move electrons only to achieve all other acceptable Lewis structures. For sulfuric acid, there are 4:

Several resonance forms for the pyrimidine base, cytosine, are shown below:

Note that the atoms are kept in fixed locations; only electrons are moved.

The following structure is NOT a resonance form, because a H atom has been moved:

This is an isomer of cytosine. Isomers are different arrangements of atoms; resonance forms are different arrangements of electrons.

Refinements

When the central atom of a species is from period > 3, it can exceed the octet. The best structure for such a species is usually considered to be the one in which sufficient double bonds are drawn to the central atom from the terminal atoms to give the central atom a formal charge of zero. The following structures would thus be drawn for SO₄^2- and ClO₄^-: