[sapta]

if i am asked to calculate the O.N. of an element in a compound that contains 2 or more atoms of the element,shud i give the average or the individual O.N. of the atoms?

for example,calculate the O.N. of Fe in Fe₃O₄.

[Mitch]

average sounds like the best idea.

[sdekivit]

better: you must use the average. Another example, how would you state the oxidation number  for S in  S4O6(2-)?

[Borek]

For me average is - educationally - wrong.

Putting aside question whether to use oxidation numbers or not, as the debates are hot (and there are more arguments against, then for), if you decide to use them they are defined as the charge of the atom in the compound.

So first you are explaining to your students that charge is integer and can't be divided, then you make them calculate oxidation number of Fe in Fe₃O₄ as 2.66.

No better way of creating confusion as giving contradicting information through the reliable sources (which loose their credibility in the process).

That reliable source is a teacher in case you missed that one  

No wonder some students hate chemistry.

Edit/sidenote: such problems are one of the reasons to NOT use oxidation numbers at all.

[xiankai]

i agree.

oxidation numbers should just be a basic stepping step for starting students until they're competent enough to know more complex bondings.

[Donaldson Tan]

for example,calculate the O.N. of Fe in Fe₃O₄.

Fe₃O₄ is otherwise known as Fe₂O₃.FeO

does that solve your problem?

[sapta]

Fe₃O₄ is otherwise known as Fe₂O₃.FeO

does that solve your problem?

oh yes.one of my books has written it Fe(FeO₂)₂.is that right also?

[Borek]

oh yes.one of my books has written it Fe(FeO₂)₂.is that right also?

In a way... That's the same as treating the compound as a salt of HFeO₂ acid - not that it exist, however, Fe(III) is slightly amphoteric and can be dissolved in hot and concentrated NaOH solution.