What is the oxidation number of hydrogen in most compounds?

The thing to remember about oxidation numbers is that they are assigned with the electronegativity of the atoms in mind.

In order to assign an oxidation number to an atom bonded to another atom, you must assume that the more electronegative of the two atoms "takes" all the bonding electrons.

Hydrogen, as you know, has a single electron surrounding its nucleus, which consequently acts as its sole valence electron.

As a result, hydrogen can only form single bonds with other atoms because it can only share one electron. Therefore, any atom bonded to hydrogen that is more electronegative than hydrogen will "take" this valence electron for itself.

This will give hydrogen a #color(blue)(+1)# oxidation number, since it only "lost" #1# electron to the more electronegative atom.

Keep in mind that when hydrogen is bonded to less electronegative atoms, such as in metal hydrides, it "takes" the bonding electron shared by the other atom for itself.

This gives it a #color(blue)(-1)# oxidation number.

Take, for example, hydrogen in water, #"H"_2"O"#. The two hydrogen atoms are bonded to an oxygen atom, which is significantly more electronegative than hydrogen.

This means that each of the two hydrogen atoms will have a #color(blue)(+1)# oxidation number, since they each "lose" their bonding electron. Consequently, oxygen will have a #color(blue)(-2)# oxidation number.

#stackrel(color(blue)(+1))("H")_ 2 stackrel(color(blue)(-2))("O")#

In sodium hydride, on the other hand, hydrogen is more electronegative than sodium, so it will have a #color(blue)(-1)# oxidation number. Consequently, sodium will have a #color(blue)(+1)# oxidation number.

#stackrel(color(blue)(+1))("Na") stackrel(color(blue)(-1))("H")#

The most common oxidation number of hydrogen will be #color(blue)(+1)#, since that's the oxidation state it has in most of the compounds it forms.

Oxidation Numbers

It is often useful to follow chemical reactions by looking at changes in the oxidation numbers of the atoms in each compound during the reaction. Oxidation numbers also play an important role in the systematic nomenclature of chemical compounds. By definition, the oxidation number of an atom is the charge that atom would have if the compound was composed of ions.

1. The oxidation number of an atom is zero in a neutral substance that contains atoms of only one element. Thus, the atoms in O2, O3, P4, S8, and aluminum metal all have an oxidation number of 0.

2. The oxidation number of simple ions is equal to the charge on the ion. The oxidation number of sodium in the Na+ ion is +1, for example, and the oxidation number of chlorine in the Cl- ion is -1.

3. The oxidation number of hydrogen is +1 when it is combined with a nonmetal as in CH4, NH3, H2O, and HCl.

4. The oxidation number of hydrogen is -1 when it is combined with a metal as in. LiH, NaH, CaH2, and LiAlH4.

5. The metals in Group IA form compounds (such as Li3N and Na2S) in which the metal atom has an oxidation number of +1.

6. The elements in Group IIA form compounds (such as Mg3N2 and CaCO3) in which the metal atom has a +2 oxidation number.

7. Oxygen usually has an oxidation number of -2. Exceptions include molecules and polyatomic ions that contain O-O bonds, such as O2, O3, H2O2, and the O22- ion.

8. The elements in Group VIIA often form compounds (such as AlF3, HCl, and ZnBr2) in which the nonmetal has a -1 oxidation number.

9. The sum of the oxidation numbers in a neutral compound is zero.

H2O: 2(+1) + (-2) = 0

10. The sum of the oxidation numbers in a polyatomic ion is equal to the charge on the ion. The oxidation number of the sulfur atom in the SO42- ion must be +6, for example, because the sum of the oxidation numbers of the atoms in this ion must equal -2.

SO42-: (+6) + 4(-2) = -2

11. Elements toward the bottom left corner of the periodic table are more likely to have positive oxidation numbers than those toward the upper right corner of the table. Sulfur has a positive oxidation number in SO2, for example, because it is below oxygen in the periodic table.

SO2: (+4) + 2(-2) = 0

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In a given compound that contains three elements (e.g. a metal, a non-metal, and hydrogen), how can it be determined whether hydrogen is connected to the metal or the non-metal so that its oxidation number can be assigned accordingly?

ringo

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asked May 10, 2015 at 5:37

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You can look at the electronegativity of the atom Hydrogen is bonded too.

For instance, in NaH the electronegativity of H is greater than that of Na (~2.1 as opposed ~.9) so H two would have an oxidation state of -1)

In HCl, we observe that Cl is more electronegative so it will have an oxidation state of -1 and H will have an oxidation state of +1

In borane (even though B is a non-metal) each hydrogen displays an oxidation state of -1 (again electronegativity of H is 2.1 as opposed 2.0 of B)

Hope this makes sense.

answered May 10, 2015 at 6:23

getafixgetafix

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When hydrogen is with a metal its oxidation number is -1 When it is with a non metal element its oxidation number is +1 In this case most probably it should be -1

answered May 10, 2015 at 5:54

SulochaneSulochane

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