Is it possible to ionize a molecule?

[menergyam]

If I have O2, and I want to make it positively charged, don't I have to break the O2 bond and then ionize the two oxygen elements to get two positively charged Oxygen atoms? Is it possible to take away an electron from O2 and still have the O2 be bonded? It is not possible right?

 

[alxm]

It's possible. Singly-ionized O₂ will still bond. Even singly-ionized H₂ bonds. It's more a question of whether you excite electrons into the anti-bonding $$\pi^\star$$ orbital, or transfer a large amount of vibrational energy to the molecule while ionizing it.

That said, O₂⁺ won't stay ionized for very long if it has the chance to grab an electron from some other, less electronegative, atom.

 

[menergyam]

Cool. So to calculate the first ionization for O2, would it be the same as the first ionization for O? I really don't think its that simple. There must be a different formula, right?

 

[alxm]

Cool. So to calculate the first ionization for O2, would it be the same as the first ionization for O? I really don't think its that simple. There must be a different formula, right?

Yes, the IP of O₂ should be smaller than for O. This is easily predicted since the molecule has more electrons to counter the charge, and secondly, the charge can be distributed over two atoms instead of one. (And it's a general chemical principle that the more distributed a charge is, the more stable it is, i.o.w. less energy)

Ionization potentials (with the single exception of the hydrogen atom) can't be calculated using any simple formula. You need the quantum-mechanical description of the system. It's roughly equal to the energy of the HOMO (Highest occupied molecular orbital) when calculated using the Hartree-Fock method (Koopmans' theorem), due to some rather nifty error cancellations.