Ozone (\(\text{O}_3\)) is a molecule composed of three oxygen atoms. Analysis of the ozone structure reveals that its bonding cannot be accurately described by a single chemical drawing. The molecule exhibits chemical resonance, meaning its true structure is an average of multiple hypothetical forms. Understanding this resonance is necessary to explain ozone’s physical properties, including the length and strength of its bonds.
Defining Chemical Resonance
Chemical resonance is a concept used when a single Lewis structure is insufficient to describe a molecule’s electron arrangement. A Lewis structure is a diagram representing the bonding of atoms and lone pairs of electrons. The problem arises because electrons are often shared across multiple atoms, a phenomenon known as electron delocalization. This means electrons are not confined to a single bond but spread their influence across three or more atoms.
Lewis structures only allow electrons to be shown as belonging to a specific bond (single, double, or triple). For molecules that exhibit resonance, the actual electronic structure is represented by a “resonance hybrid.” This hybrid is a blend of all possible Lewis structures, which are called resonance contributors or forms. The hybrid is the molecule’s true, time-averaged structure at all times, not a rapid switching between forms. The resonance forms are simply a tool to visualize electron delocalization.
Visualizing the Ozone Structure
The ozone molecule consists of a central oxygen atom bonded to two outer oxygen atoms, forming a bent shape. Drawing a single Lewis structure for \(\text{O}_3\) requires placing 18 valence electrons around the three atoms. To satisfy the octet rule, the structure must include one double bond and one single bond between the central atom and the two outer atoms. This arrangement results in a separation of formal charge, with the central atom positive and the single-bonded outer atom negative.
The limitation of a single drawing is that there are two equivalent ways to draw this structure, which serve as the primary resonance forms for \(\text{O}_3\). These forms are identical in energy and stability, contributing equally to the molecule’s overall structure. The actual bonding is best represented by a resonance hybrid, where the \(\pi\)-electrons are delocalized across all three oxygen atoms. This delocalization is often drawn using a dashed line to indicate a partial, or “one-and-a-half,” bond between the central oxygen atom and each outer oxygen atom.
Physical Effects of Resonance
The concept of resonance has measurable, physical consequences that confirm electron delocalization in the ozone molecule. If ozone had a fixed single bond and a fixed double bond, as depicted in a single Lewis structure, the two oxygen-oxygen bonds would have different lengths. A typical oxygen-oxygen single bond is approximately 148 picometers (pm), while a double bond is shorter, closer to 121 pm.
Experimental measurements of the \(\text{O}_3\) molecule show that both oxygen-oxygen bonds are identical in length, measuring approximately 127.8 pm. This measured length is intermediate between the characteristic lengths of a single and a double bond. This provides empirical proof that the true structure is the resonance hybrid, where the bond order is 1.5.
The delocalization of electrons also results in greater stability for the \(\text{O}_3\) molecule than predicted for a structure with localized bonds. This added stability, called resonance stabilization, means the molecule exists in a lower energy state because the negative charge density is spread out over a larger area.