Which of the following odd electron species is planar?

Which of the following molecules or ions possesses electrovalent,covalent,and coordinate bonds at the same time?

Fill in the blanks given below:
$(i)$ The electronic configuration of noble gases is .......... .
$(ii)$ The bond formed by electrostatic attraction between a cation and an anion is called ........... .
$(iii)$ The octet rule is also known as ............ .
$(iv)$ The equilibrium distance between the centers of two bonded atoms in a molecule is called .......... .

Which of the following molecules does not obey the octet rule?

Solid $BeH_2$ and solid $BeCl_2$ differ in

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