Read MO Diagrams Before Ranking Bond Order or Stability
Bond order = (bonding electrons − antibonding electrons)/2, and every ranking or 'fractional bond order' question is just this formula applied to different electron counts across a diatomic and its ions.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Removing or adding an electron changes bond order predictably
removing an electron from an antibonding MO (as in NO → NO⁺, since NO's highest electron sits in π*) also increases bond order — the direction of the shift depends on which type of orbital (bonding or antibonding) held the removed electron, not simply on whether an electron was added or removed.
Fractional bond order comes from odd electron counts in antibonding orbitals
O²⁻ has bond order 1.5, O2²⁻ has bond order 1.0 — each extra antibonding electron beyond O2's own 2 subtracts 0.5 from bond order. Any species where the antibonding electron count is odd (relative to a filled/half-filled pattern) gives a fractional bond order; this is a fast way to scan a list of species for the fractional-bond-order ones without drawing every diagram.
Stability tracks bond order, with N₂ as the classic exception to memorise
For most diatomics, higher bond order means greater stability (and shorter bond length) — but N₂ itself has the HIGHEST bond order (3) of the N2/N₂+/N₂-/N2²⁻ series and both N²⁺ and N²⁻ have equal, lower bond order (2.5), making N₂ uniquely the most stable of that whole series, unlike O₂ where O²⁺ is actually more stable than neutral O₂.
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- ✓Bond order = (bonding e- − antibonding e-)/2 — recompute from the actual MO filling, don't guess from formula.
- ✓Removing an electron from an antibonding orbital increases bond order (e.g. NO→NO⁺); removing from a bonding orbital decreases it (opposite for some series — check which orbital type is affected).
- ✓A fractional bond order signals an odd number of antibonding electrons relative to the parent diatomic.
- ✓N₂ is the bond-order/stability maximum of its own ion series; O₂, by contrast, has O²⁺ as MORE stable than neutral O₂ — these two series behave oppositely, don't assume the same pattern for both.
Don't assume removing an electron always increases stability the same way across different diatomics — check whether the removed electron came from a bonding or antibonding orbital in THAT specific species' MO diagram; N₂ and O₂ behave oppositely on ionisation.
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