Which of the following facts regarding bond order is not valid ?

Options

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 decrease.

Explanation

To determine which of the following facts regarding bond order is not valid, we need to analyze each statement related to bond order.

### Step-by-Step Solution:

1. **Understanding Bond Order**:
- Bond order is defined as the number of bonds between two atoms in a molecule. For example, in propene (C3H6), the bond order between two carbon atoms that are connected by a double bond is 2 (one sigma bond and one pi bond).

2. **Evaluating the Statements**:
- **Statement 1**: "Bond order is given by the number of bonds between two atoms in a molecule."
- This statement is true. Bond order indeed represents the number of bonds between two atoms.

- **Statement 2**: "With increase in bond order, bond enthalpy of the molecule decreases."
- This statement is false. As bond order increases, the bond enthalpy (the energy required to break the bond) actually increases. More bonds mean stronger interactions, requiring more energy to break them.

- **Statement 3**: "Isoelectronic molecules and ions have identical bond order."
- This statement is true. Isoelectronic species have the same number of electrons, and thus, their bonding and antibonding molecular orbitals will be filled similarly, leading to the same bond order.

- **Statement 4**: "With increase in bond order, bond length decreases."
- This statement is true. As bond order increases, the strength of the bond increases, causing the atoms to be pulled closer together, which results in a shorter bond length.

3. **Conclusion**:
- The only incorrect statement among the options provided is **Statement 2**. Therefore, the answer to the question is that the statement "With increase in bond order, bond enthalpy of the molecule decreases" is not valid.

### Final Answer:
**The statement that is not valid regarding bond order is: "With increase in bond order, bond enthalpy of the molecule decreases."**

---