Question

(a) Describe the molecule chlorine dioxide, \(\mathrm{ClO}_{2}\), using three possible resonance structures. (b) Do any of these resonance structures satisfy the octet rule for every atom in the molecule? Why or why not? (c) Using formal charges, select the resonance structure(s) that is (are) most important.

Short answer

(a) The three possible resonance structures for ClO₂ are: 1. Chlorine single bonded to both oxygens with a lone pair on chlorine. 2. Chlorine double bonded to one oxygen and single bonded to the other, with oxygens and chlorine having lone pairs. 3. Same as structure 2 but with switched double and single bonded oxygens. (b) Resonance Structures 2 and 3 satisfy the octet rule for all atoms, as both oxygens have 8 electrons in their outer shells and chlorine has an expanded octet with 10 electrons due to being in the third period. (c) Using formal charges, Resonance Structures 2 and 3 have the lowest total formal charges (0), making them the most important. The actual molecule represents an average of these two resonance structures.

Step-by-step solution

Draw Three Possible Resonance Structures for ClO2

To draw the resonance structures, let's start by counting the total number of valence electrons available in the molecule. Chlorine has 7 valence electrons, and oxygen has 6 valence electrons: 7 (Cl) + 2(6) (Oxygen) = 19 valence electrons. Now, we can proceed to draw the resonance structures: Resonance Structure 1: Chlorine forms a single bond with both oxygens and has a lone pair of electrons. Single bonds: 2 electrons each (total of 4 electrons) Lone pair on Cl: 2 electrons Remaining electrons: 13 electrons (distributed among the oxygen atoms as lone pairs) Resonance Structure 2: Chlorine forms a double bond with one oxygen atom and a single bond with the other oxygen atom. Each oxygen atom has two lone pairs (total 8 electrons) and Chlorine has 3 lone pairs (6 electrons). Resonance Structure 3: Switch the double bonded and single bonded oxygens in Resonance structure 2.

Check if Octet Rule is Satisfied for All Atoms

The octet rule states that every atom (except hydrogen) should have 8 electrons in its outer shell either by sharing or having lone pairs. Resonance Structure 1: Each oxygen atom has 8 electrons (2 shared and 6 as lone pairs). Chlorine has only 6 electrons in its outer shell (4 shared and 2 as a lone pair). The octet rule is not satisfied for chlorine. Resonance Structure 2 and 3: Both oxygen atoms have 8 electrons: one with 4 shared +4 as lone pairs and the other with 2 shared + 6 as lone pairs. Chlorine, however, has an expanded octet of 10 electrons (6 shared and 4 as lone pairs or in bonding electrons), which is acceptable for atoms from the third period or below (like Chlorine). So, both Resonance Structure 2 and 3 satisfy the octet rule.

Use Formal Charges to Select the Most Important Resonance Structure(s)

Formal charge is calculated as follows: Formal charge = (valence electrons) - (non-bonding electrons) - 1/2(bonding electrons). Resonance Structure 1: Formal charge on Cl: \(= 7 - 2 - 1/2(4) = +1\) Formal charge on both Oxygens: \(= 6 - 6 - 1/2(2) = 0\) Total formal charge: +1 Resonance Structure 2: Formal charge on Cl: \(= 7 - 0 - 1/2(10) = 0\) Formal charge on doubly bonded O: \(= 6 - 4 - 1/2(4) = 0\) Formal charge on singly bonded O: \(= 6 - 6 - 1/2(2) = 0\) Total formal charge: 0 Resonance Structure 3: Same as Resonance Structure 2 (atoms are just switched in positions). The resonance structures with the lowest formal charges are more important, so Resonance Structure 2 and 3 are the most important. In reality, the molecule distributes the electrons in a way that represents an average of these two resonance structures.