more on oxidation numbers

[Martoman]

I don't know why i'm getting these wrong.

. So oxygen is -2. Cl is -1 so K = +9 but doesn't K = +1 so Cl = +7? Which order? How do we know which order?

Similarly O = -2 K = +4 Or K = 1, O = -0.5?

[monokekie]

arhh, look at KClO4 this way, K+ and ClO4- =]

[Mao]

Tricky.

In the first case, Cl is +7 (double bonded to three oxygens, single bonded to an O^-, structure is similar to sulfate and phosphate)

In the second case, the O2 is present as superoxide O₂^-, and each are -0.5

This draws on knowledge that ionisation energies for valence shells are much lower than ionisation energies for inner shells. From an energy perspective, it is much easier to remove all of the valence shell from Cl than to tap into the inner shell of K (and similarly, oxygen atoms are depleted of electrons because it is very hard to remove the second electron from K)

[Martoman]

ah ok. so there is some logic to oxidation numbers.... that is they have some grounding in the real world. So we do before all others assign group 1 their +1 first then movie onto oxygen and such. ok.