Question

Bond order of $Cl-O$bond in $Cl{{O}_{4}}^{-}$ ion and the effective charge on each oxygen atom respectively are:
A. 1.75, -1
B. 1.75, -0.25
C. 1.5, -0.5
D. 2.0, -0.5

Answer 1

In molecular orbital theory bond order is defined as half the difference between the number of bonding electrons and the number of antibonding electrons which can be shown as:
$B.O=\dfrac{number\ of\ bonding\ electron-number\ of\ antibonding\ electron}{2}$

Complete answer:
Bond order is directly proportional to the strength of the bond i.e. higher the bond order more the strength of bond is. The effective nuclear charge is defined as the positive charge experienced by an electron in an atom here the term effective is used because the shielding effect of negatively charged electrons prevents higher orbital electrons from experiencing the full nuclear charge of the nucleus due to the repelling effect of inner-layer electrons. The effective nuclear charge experienced by the electron is also known by the name core charge. It is possible to determine the strength of the nuclear charge by knowing the oxidation number of the atom.
Bond order can also be calculated from another formula which is shown as follows:
$Bond\ order=Valency\ of\ peripheral\ atoms+\dfrac{charge\ on\ acid\ radical}{tota\operatorname{l}\ number\ of\ peripheral\ atoms}$
$Cl{{O}_{4}}^{2-}=2+(\dfrac{-1}{4})=\dfrac{7}{4}$= 1.75
Effective nuclear charge of $Cl{{O}_{4}}^{-}$is -1.

Hence we can say that option A is the correct answer.

Note:
The combining capacity of an atom is known as its valency. The number of bonds that an atom can form as part of a compound is expressed by the valency of the element. Valency is different from the oxidation number. For example valency of nitrogen is 3 whereas it can have oxidation numbers from -3 to +5.