Choose the correct sequence for the geometry of the given molecules: Borazone,Borazole,$B_3O_6^{3-}$,$Fe_2Cl_6$,trimer of $FCN$. ['$P$' stands for planar and '$NP$' stands for non-planar]

The bonds present in $N_2O_{5(s)}$ is/are:
$I$. Ionic
$II$. Covalent
$III$. Coordinate

Which of the following species does not exist?

Consider the following species and select the incorrect statement $:-$ $NH_3, PCl_4^{\oplus}, NH_2^{-}, SO_2, SO_3, BF_3, PCl_3, IF_7$

Identify the molecule / ion in which the ratio of $\sigma$ to $\pi$ bonds is $3: 2$.

Fill in the blanks given below:
$(i)$ The attractive force which holds atoms,molecules,and ions together is called ......... .
$(ii)$ First of all,............ and ............ scientists provided detailed information about valency.
$(iii)$ In the structure of an atom,when only the electrons of the outermost shell participate,they are known as ............ .
$(iv)$ The symbol used to represent valence electrons in an atom is called ........ .