The correct bond order of the $B-F$ bond in the $BF_3$ molecule is:

What is a resonance structure? Why are resonance structures required? Explain with a suitable example.

Resonance in $X_2Y$ can be represented as shown in the image. The enthalpy of formation of $X_2Y$ $(X \equiv X(g) + \frac{1}{2} Y = Y(g) \rightarrow X_2Y(g))$ is $80 \ kJ \ mol^{-1}$. The magnitude of resonance energy of $X_2Y$ is $......... \ kJ \ mol^{-1}$ (nearest integer value). Given: Bond energies of $X \equiv X, X=X, Y=Y$ and $X=Y$ are $940, 410, 500$ and $602 \ kJ \ mol^{-1}$ respectively. Valence $X: 3, Y: 2$.

Which of the following pairs are not resonating structures?

Given that:
$(i) \Delta_f H^o$ of $N_2O$ is $82 \ kJ \ mol^{-1}$
$(ii)$ Bond energies of $N \equiv N, N = N, O = O$ and $N = O$ are $946, 418, 498$ and $607 \ kJ \ mol^{-1}$ respectively.
The resonance energy of $N_2O$ is $...... \ kJ \ mol^{-1}$.

$(i)$ Identify the less stable resonance structure and $(ii)$ give its reason for the following pairs.