Which of the following contains both ionic and covalent bonds?

Give reasons for the following:
$A$. Covalent bonds are directional bonds while ionic bonds are non-directional.
$B$. Water molecule has bent structure whereas carbon dioxide molecule is linear.
$C$. Ethyne molecule is linear.

The total number of lone pairs of electrons in melamine is

Compare $\pi$-bond strength between $B$ and $N$ in the following compounds: $(I)$ $((CH_3)_3Si)_2N^{-}BH_2$ and $(II)$ $((CH_3)_3C)_2N^{-}BH_2$.

Sodium bicarbonate $(NaHCO_3)$ has which of the following types of bonds?

Which of the following molecules has the weakest $(p \pi - d \pi)$ back bonding?