Oxidation state of hydrogen

[HopefulMDclass2020]

Hello,

I read in TPR that for assigning oxidation states, that hydrogen has a +1 when bound to something more EN than carbon, a -1 when bound to something less EN than carbon, and a 0 when bonded to carbon.

My question is this. I know the carbon on methane (CH4) has a -4 O.S, meaning that the hydrogen have to have a +1 O.S each. This seems to contradict the text that I bolded.

Any explanation?
Thanks

---

Members don't see this ad.

 

[Cawolf]

It is -4 AFAIK.

Oxidation state is just a number to represent electron density on atoms.

Carbon is more electronegative than hydrogen, so in C-H bonds there is a slight negative polarity towards the carbon.

 

[Axes]

Just think of it like H can have OS of +1 or -1, and then just pick the right one based on the fact that the whole compound needs to sum to 0. Maybe not super erudite but probably all that's required for the MCAT

 

[chemist16]

Also keep in mind that formal oxidation state is only a rough estimate of which atom gets the electrons in a bond. So the rule is that the more electronegative atom gets the electrons. If you look up the electronegativity values for hydrogen and carbon, you'll see that H is 2.1 and C is 2.5. That's why the TPR "rule" works. In a C-H bond, the electrons are roughly shared equally since they have roughly equivalent electronegativities. In a H-Cl bond, chlorine is very electronegative and pulls the electron density towards itself, leaving you with, essentially, a proton. However, as you can probably imagine, oxygen is more electronegative than chlorine so even though the "formal" oxidation state of H in both cases is +1, the actual oxidation state of H in an O-H bond is actually higher than that of H in a H-Cl bond. That's how formal oxidation states work, if you want an intuitive explanation of it.

 

[chemtopper]

remember H bonded to metals ...it is -1 and bonded to non metals ...it is +1 ..however in BH3 ...H is more electronegative then B so H is -1 ..