57) CaO and NaCl have the same crystal structure and approximately the same ionic radii. If U is the lattice energy of NaCl, the approximate energy of NaCl is:

58) CH₃⁻ is an isostere of:

59) The nature of π bond in perchlorate ion is:

60) In the anion HCOO⁻, the 2 C−O bonds are found to be of equal length. What is the reason for it?

61) In which of the following molecules are all the bonds not equal?