Please note that several atoms follow the octet rule, i.e., they tend to achieve eight electrons in their valence shell through chemical bonding this is reflected in the Lewis structure of the molecule. The structures drawn using this theory are termed Lewis (dot) structures. The purpose of this theory is to help visualize the chemical bonding of atoms in molecules.Įlectrons are represented as dots, and each pair of bonding electrons between two atoms is shown as a line. Again, experiments show that all three C–O bonds are exactly the same.The Lewis theory of chemical bonding-although quite primitive and the most limited theory on electronic structure-does help one to determine how valence electrons are arranged around the constituent atoms in a molecule. Because we can write three identical resonance structures, we know that the actual arrangement of electrons in the carbonate ion is the average of the three structures. This gives rise to three resonance forms of the carbonate ion. All oxygen atoms, however, are equivalent, and the double bond could form from any one of the three atoms. One oxygen atom must have a double bond to carbon to complete the octet on the central atom. Experiments show, however, that both N–O bonds in \(,\) provides a second example of resonance: A double bond between two atoms is shorter (and stronger) than a single bond between the same two atoms. If nitrite ions do indeed contain a single and a double bond, we would expect for the two bond lengths to be different. The electrons involved in the N–O double bond, however, are in different positions: You may have noticed that the nitrite anion in can have two possible structures with the atoms in the same positions. Also, it places the least electronegative atom in the center, and the negative charge on the more electronegative element (Guideline 4). However, the first arrangement of atoms is preferred because it has the lowest number of atoms with nonzero formal charges (Guideline 2). Note that the sum of the formal charges in each case is equal to the charge of the ion (–1). Possible Lewis structures and the formal charges for each of the three possible structures for the thiocyanate ion are shown here: The formal charges present in each of these molecular structures can help us pick the most likely arrangement of atoms. 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:Ĭomparing 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).Īs 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 –. 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. To see how these guidelines apply, let us consider some possible structures for carbon dioxide, CO 2. When we must choose among several Lewis structures with similar distributions of formal charges, the structure with the negative formal charges on the more electronegative atoms is preferable.Lewis structures are preferable when adjacent formal charges are zero or of the opposite sign.
If the Lewis structure must have nonzero formal charges, the arrangement with the smallest nonzero formal charges is preferable.A molecular structure in which all formal charges are zero is preferable to one in which some formal charges are not zero.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: 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.
The arrangement of atoms in a molecule or ion is called its molecular structure. Using Formal Charge to Predict Molecular Structure
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