Predicting Oxidation Numbers

[Caleb]

I have been reading in my chemistry book about oxidation numbers. For the elements that have several oxidation numbers, how can you predict which one it will use? I know it has something to do with the difference of the electronegitivities between the elements, for example:

Since Nitrogen has a higher electronegitivity than hydrogen, the nitrogen atom will gain a negitive oxidation number and the hydrogen will gain a positive oxidation number. Since the electronegitivities are far enough apart, nitrogen can assume its highest negitive oxidation number, which happens to be -3. So in order for the sum of the oxidation numbers of the elements to equal zero, there must be three hydrogen bonded with the nitrogen. Thus creating ammonia.

How can you tell if the electronegitivities are "far enough apart" to get the desired oxidation number?

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Consecutive posts merged

When does the electronegitivity differance start making compouds ionic?

[Guest Nerddoc]

I have been reading in my chemistry book about oxidation numbers. For the elements that have several oxidation numbers, how can you predict which one it will use? I know it has something to do with the difference of the electronegitivities between the elements, for example:

Since Nitrogen has a higher electronegitivity than hydrogen, the nitrogen atom will gain a negitive oxidation number and the hydrogen will gain a positive oxidation number. Since the electronegitivities are far enough apart, nitrogen can assume its highest negitive oxidation number, which happens to be -3. So in order for the sum of the oxidation numbers of the elements to equal zero, there must be three hydrogen bonded with the nitrogen. Thus creating ammonia.

How can you tell if the electronegitivities are "far enough apart" to get the desired oxidation number?

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Merged post follows:

Consecutive posts merged

When does the electronegitivity differance start making compouds ionic?

 

Hey,

 

Great question. I never quite thought of oxidation numbers that way, but now that you mention it- it makes sense. If it helps your reasoning, the electronegativity difference of greater than or equal to two between a metal and a non metal produces a ionic bond between them. Anything less than that with produce either a polar covalent or covalent bond. Hope this helped!

[Horza2002]

Hey,

 

What your talkign about here is formal oxidation states. You have to remember that they are human idea and especially in nonmetal systems dont really have any meaning. Usually you just have to say that the more electronegative will be more negative and it doesnt matter how much by.

 

Oxidation states only really become important when considering metal complexe since the oxidation state if the metal ion ha extremely large effects on the stability and reactivity of the complex. In that case, most metals are electropoitive and so end up with a formal positive charge.

[Caleb]

Usually you just have to say that the more electronegative will be more negative and it doesnt matter how much by.

 

 

The definition of an oxidation number according to "General Chemistry" by Whitten and Gailey states:

Quote - "The oxidation number or oxidation state of an element is the number of electrons gained or lost by an atom of that element when it forms an ionic compound. In covalent compounds... In such cases positive and negative oxidation numbers indicat shifts (not transfers) of electron density from one atom toward another. The more electronegative element is assigned a negitive oxidation number, while the less electronegative element is assigned a positive oxidation number."

 

As an example:

 

Fluorine has an electronegitivty of 4.0

 

Phosphorous has an electronegitivity of 2.1

 

The differance is roughly two. So, the electronegitivities are far enough apart for F to "max out" the P. Because F wants the electrons so bad, it will try to make P as positive as possible. But if you take a lower halogen, like Iodine with a electronegitivity of 2.5 and react it with P, you get phosphorous triiodide instead (PI3) or diphosphorus tetraiodide (P2I4) instead of phosphorous pentaiodide (PI5), which is possible but rare. (According to wikipedia, under Phosphorus triiodide)

 

What I want to know is, is there a way to calculate or generalize what oxidation number a compound will use under "normal" conditions. What I mean by normal is, not something like creating some nearly impossible compound at 100,000 atm and at 100,000^50 K. As close to normal pressure and temperature.

[Horza2002]

Whether you get PI3, P2I4 or PI5 depends on a lot of other factors other than possible oxidation states. You have to consider the entropy and enthalpy of the reaction as well. However, each element has certain oxidation states which it is most stable; you'll find these in any main group chemistry book.

 

But as i said before electronegative elements favour negative oxidation states so that they get a full outer shell of electrons where as electropositive elements favour positive oxidation states again so they have a full valence shell.

[mississippichem]

Its often safe to assume that Oxygen is in the 2- oxidation state, Hydrogen bonded to metals is in the 1- oxidation state and bonded to non-metals is in the

+1 oxidation state. Univalent halogens are almost always 1-. From this it's easy to deduce the oxidation states for other atoms as many compounds include O, H, Cl, Br... For example:

 

 

PF5:

overall charge: 0

F: (-1)5 = -5

so (-5) + (x) = 0

x = 5...P is in 5+ oxidation state.

[JN.]

Its often safe to assume that Oxygen is in the 2- oxidation state,  Hydrogen bonded to metals is in the 1- oxidation state and bonded to non-metals is in the

+1 oxidation state.  Univalent halogens are almost always 1-.  From this it's easy to deduce the oxidation states for other atoms as many compounds include O, H, Cl, Br... For example:

 

 

PF5:

overall charge: 0

F: (-1)5 = -5

so (-5) + (x) = 0

x = 5...P is in 5+ oxidation state.

 

And we can assume that alkali metals have always 1+ and alkali earth elements have 2+. Oxygen can vary from 1- to 2-, although it's more common to be 2-.