Number of $\pi$ electrons in cyclobutadienyl anion $({C_4}{H_4})^{2-}$ is

What is the formal charge on the $N$ atom in the $NH_4^{+}$ ion?

Bonds present in $CuSO_4 \cdot 5H_2O$ are

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

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

Which of the following have $X-O-X$ linkage? (where $X$ is the central atom):
$(i) \ Cr_2O_7^{2-}$
$(ii) \ S_2O_3^{2-}$
$(iii) \ \text{Pyrosilicate}$
$(iv) \ \text{Hyponitrous acid}$