In the reaction $SnCl_2 + 2Cl^{-} \to SnCl_4$,the Lewis acid is:

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.

Arrange the following species in the increasing order of lone pairs of electrons:
$(A)$ $CO$
$(B)$ $NO_2^-$
$(C)$ $NF_3$
$(D)$ $CO_3^{2-}$

Identify the set of molecules/ions that are isoelectronic.

Fill in the blanks given below:
$(i)$ The two theories based on the principles of quantum mechanics are ........ and ......... .
$(ii)$ The electronic configuration of carbon in the excited state is ........... .
$(iii)$ Through the combination of atomic orbitals,....... are formed.
$(iv)$ The ${\rm{CH}}_4$ molecule has a .......... shape.

Total number of compounds / ions which have at least two bond lengths same out of $CO_3^{2-}, HCOOH, S_2O_3^{2-}, NO_2^{-}$