Properties and Structure of Ozone Molecule

To determine the correct statements regarding the ozone ($\text{O}_3$) molecule, we analyze each statement individually:

1. Structure and Bond Lengths (Statements A and D):

   • The central oxygen atom in the $\text{O}_3$ molecule has a total of 6 valence electrons. In its Lewis structure, it forms a double bond with one terminal oxygen atom and a coordinate (dative/single) bond with the other terminal oxygen atom, retaining one lone pair of electrons: $\text{O}=\overset{+}{\text{O}}-\text{O}^- \longleftrightarrow \text{O}^--\overset{+}{\text{O}}=\text{O}$
   • The steric number of the central oxygen atom is: $\text{Steric Number} = 2\,(\sigma\text{-bonds}) + 1\,(\text{lone pair}) = 3$ This corresponds to $sp^2$ hybridization with a trigonal planar electron-pair geometry. Because of the presence of the lone pair, the molecular geometry is bent (angular or V-shaped), with a bond angle of approximately $116.8^\circ$. Thus, statement (D) is correct.
   • Due to resonance, the two $\text{O}-\text{O}$ bonds are equivalent, resulting in a partial double-bond character (bond order of $1.5$) for both bonds. As a result, both $\text{O}-\text{O}$ bond lengths are identical and equal to approximately $128\text{ pm}$, intermediate between a single bond ($\approx 148\text{ pm}$) and a double bond ($\approx 121\text{ pm}$). Thus, statement (A) is correct.

2. Thermodynamics of Thermal Decomposition (Statement B):

   • The decomposition of ozone into dioxygen is represented by: $2\text{O}_3(g) \longrightarrow 3\text{O}_2(g)$
   • Ozone is thermodynamically less stable than molecular oxygen. Its standard enthalpy of formation from oxygen is positive ($\Delta_f H^\circ \approx +142.7\text{ kJ mol}^{-1}$).
   • Consequently, the decomposition of ozone to oxygen is an exothermic process ($\Delta H < 0$). This enthalpy change, combined with the increase in entropy ($\Delta S > 0$ as 2 moles of gas yield 3 moles of gas), results in a large negative Gibbs free energy change ($\Delta G < 0$), making the decomposition spontaneous and strongly exothermic. Thus, statement (B) is incorrect.

3. Magnetic Nature (Statement C):

   • The total number of electrons in an $\text{O}_3$ molecule is $3 \times 8 = 24$ (including 18 valence electrons).
   • In its ground electronic state, all electrons are paired across bonding, non-bonding, and molecular orbitals, leaving no unpaired electrons. Therefore, $\text{O}_3$ is diamagnetic. Thus, statement (C) is correct.

Hence, the correct statements are A, C, and D.