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Which of the following facts regarding bond order is not valid?
(A) Bond order is given by the number of bonds between the two atoms in a molecule
(B) With increase in bond order, bond enthalpy of the molecule decreases
(C) Isoelectronic molecules and ions have identical bond orders
(D) With increase in bond order, bond length decreases

Last updated date: 16th Jun 2024
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Hint: A bond order determines the number of bonds present in the molecule which also gives us information regarding bond length, stability and enthalpy values.

Complete step by step answer: The bond order is defined as the number of bonds between the two atoms in a molecule. In other words, the number of bonds attached to the compound gives the bond order. Hence, the first statement is valid.
When bond order increases, the stability of the compound increases since the bond becomes stronger so high energy is required to break the bond hence the bond enthalpy increases. Hence, the second statement is not valid.
The molecules which have the same number of electrons are called isoelectronic molecules. Thus, the same number of electrons means there will be no change in bond order. Hence, the third statement is valid.
It is well known that the bond order and bond length are inversely related. Higher the bond order, stronger will be the pull between the two atoms and hence shorter the bond length. For example, a triple bond (bond order is 3) is shorter than a single bond (bond order is 1). Thereby, as bond order increases the bond length decreases. Hence, the fourth statement is valid.

So, the correct option is C.

Note: A bond order increases across a period and decreases down a group. Bond order can be determined by the formula below:
\[{\text{Bond Order = }}\dfrac{{\left( {{\text{No}}{\text{. of electrons in anti bonding M}}{\text{.O}}} \right){\text{ - }}\left( {{\text{No}}{\text{. of electrons in bonding M}}{\text{.O}}} \right)}}{2}\]