The Cl − O bond order in per chlorate ion

The structure of perchlorate ion is shown below. 

The Cl − O bond order in per chlorate ion is

$\mathrm{Bond} \mathrm{order} =\frac{\mathrm{Total} \mathrm{no}. \mathrm{of} \mathrm{bonds} \mathrm{between} \mathrm{Cl} \& \mathrm{O}}{\mathrm{No}. \mathrm{of} \mathrm{canonical} \mathrm{structure}}$

$\therefore \mathrm{Bond} \mathrm{order}=\frac{7}{4}=1.75$