Choose the correct order of $C-C$ bond length in the given compounds.

Which one of the following molecules contains both ionic and covalent bonds?

The ionization enthalpy of $Na^{+}$ formation from $Na_{(g)}$ is $495.8 \ kJ \ mol^{-1},$ while the electron gain enthalpy of $Br_{(g)}$ is $-325.0 \ kJ \ mol^{-1}$. Given the lattice enthalpy of $NaBr_{(s)}$ is $-728.4 \ kJ \ mol^{-1}$. The energy for the reaction $Na_{(g)} + Br_{(g)} \rightarrow NaBr_{(s)}$ is $(-) \dots \dots \dots \dots \dots \times 10^{-1} \ kJ \ mol^{-1}$.

Pick out the isoelectronic structures from the following:
$\mathop {CH_3^+}\limits_{I}$ $\mathop {H_3O^{+}}\limits_{II}$ $\mathop {NH_3}\limits_{III}$ $\mathop {CH_3^-}\limits_{IV}$

Which of the following cannot act as a Lewis base?

The total number of lone pairs of electrons in the given molecule is