AX is a covalent diatomic molecule where A and X are second row elements of the periodic table. Based on Molecular Orbital Theory, the bond order of AX is 2.5. The total number of electrons in AX is

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Step 1 — Bond order formula:

Step 2 — MO filling for second-row diatomics:
For a molecule with total electrons n:
- First 4 electrons fill  and  (cancel out)
- Next 4 fill  and  (cancel out)
- Remaining electrons fill valence MOs

For BO = 2.5, we need  in valence MOs.

Step 3 — Try n = 15 electrons:
Valence electrons = 15 − 8 (core) = 7
Filling:  → ,

Example: NO (7 + 8 = 15 electrons), BO = 2.5

Answer: 15

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