Which of the following has a different number of $p_{\pi}-d_{\pi}$ bonds compared to the other molecules?

Chemical bond implies

The correct statements from the following are:
$a$) The total number of sigma bonds present in benzene is $12$
$b$) $LiF$ is more covalent than $KF$
$c$) $SnCl_2$ is more covalent than $SnCl_4$

Who first provided an explanation for the formation of chemical bonds in terms of electrons?

The formal charges of $N_{(1)}$,$N_{(2)}$ and $O$ atoms in the structure shown below are respectively:
$:N_{(1)}=N_{(2)}=\ddot{O}:$

In the following electron-dot structure, calculate the formal charge from left to right for the nitrogen atoms: $: \ddot{N} = N = \ddot{N} :$