Oxidation Numbers

An oxidation number is a positive or negative number that indicates how many electrons an atom has gained, lost, or shared to become stable.

3,227 Questions
Oxidation Numbers

What is the oxidation number of sulfur in SO3?

+6, because the oxidation number of oxygen is -2x3=-6

Oxidation Numbers

What is the oxidation number of C in CH3COOH?

-3 on the first C and +3 on the second C.

Oxidation Numbers

What is the oxidation number of hydrochloric acid?

The oxidation number ( or state for people outside the US) is +1 for hydrogen and -1 for chlorine

Oxidation Numbers

Oxidation number of O in K2O2?


Oxidation Numbers

What is the oxidation number CH4?

In methane, also known as CH4, the carbon and hydrogen share valence electrons in a covalent bond. Each hydrogen atom shares its single electron with the carbon atom, while the carbon atom which has four valence electrons shares one electron with each of the hydrogen atoms. Carbon is more electronegative than hydrogen, so we can describe this is carbon oxidizing to -4, and hydrogen to +1. Although the electrons are shared, they spend more time closer to the carbon nucleus.


Following the normal rules (IUPAC) H is always +1 except when bonded to a metal.


If in normal cases the oxidation number of H is supposed to be +1 (except in most hydrides), then with the total of oxidation numbers of all atoms in a compound equalling the total charge thereof, the oxidation number if carbon in CH4 is -4 because:

[ OxC + 4*(+1) ] = 0, so OxC = 0 - 4*1 = -4

Some examples:

In the same way OxC can be calculated in ethane, C2H6, it is -3

and in benzene, C6H6, it is -1

In CO2: OxC = +4 , when OxO = -2, valid for all oxides, except peroxides)

Oxidation Numbers

What is the oxidation number of sulfur in Na2S2O4?

To figure this out, we first need to figure out the charge on S2O4.

We know that Na has a +1 charge, so the Na2 portion of the salt will have an overall +2 charge (2 x Na).

Provided that Na2S2O4 is a neutral molecule, we know that the charge of S2O4 must balance Na2, which has a +2 charge. Therefore, the charge on S2O4 is -2.

If the charge on S2O4 is -2, that must mean that there is an excess of 2 electrons in the molecule. We know that Oxygen usually has a charge of -2, and given that there are 4 Oxygen atoms in S2O4, the charge contributed by Oxygen is 4 x (-2) = -8. Since the charge of S2O4 is -2, we know that the total charge contributed by sulfur is +6.

+6 divided by 2 Sulfur atoms gives you the oxidation state (or number) of S: +3

Oxidation Numbers

What is the oxidation number of H in H3PO4?


Oxidation Numbers

What is the oxidation number for NH4I?

Oxidation numbers of the elements in NH4I are

N: -3

H: +1 (4 times)

I: -1


NH4+ : +1

and I- : -1

Oxidation Numbers

What causes oxidation numbers?

Oxidation is a process of loss of electron(s) from an element or ion. After the lose of electron, the element becomes positively charged or if an undergoes oxidation then its positive charge is increased. For instance Sodium metal has 11 electrons and it can easily lose one electron (removing another electron is so difficult and it needs high energy) and becomes Na(+) ion. So its oxidation number is one.

Hence, oxidation number refers number of electrons lost by an ion or element.

Reverse of oxidation is called reduction or gain of electrons by an element or ion, in which the oxidation number is decreased. So in reduction process oxidation number is decreased.

Oxidation number may be positive or negative for ions and it must be zero for an element. If it is positive then the ion is called cation and if negative then the ion is known as anion. It entirely depends on the property of losing or gaining of electrons by an element or ion.


Dr M Kanagasabapathy Ph D,

Asst. Professor, Department of Chemistry,

Rajus College, Affiliated to Madurai Kamaraj University

Rajapalayam, Tamil Nadu INDIA 626 117

Oxidation Numbers

How do you determine the oxidation number of an atom?

Follow these 5 Rules1) The oxidation number of the atoms in a neutral molecule must add up to zero, and those in a charged ion must add up to the charge on the ion.

2) Alkali metal atoms (first column in the periodic table) have oxidation number +1, alkaline earth atoms (2nd column) have oxidation number +2.

3) Fluorine always has oxidation number -1. Other halogens (next-to-last column in the periodic table) usually have oxidation number -1, except when they are bonded to oxygen or other halogens, when they can have positive oxidation numbers.

4) Hydrogen has oxidation number +1 except in metal hydrides, such as LiH or CaH2, where Rule #2 takes precedence, and hydrogen has oxidation number -1.

5) Oxygen has oxidation number -2, with two exceptions: In compounds where it is bonded to fluorine, Rule #3 takes precedence, and in compounds with O-O bonds, Rules #2 and #4 take precedence. So, for example, the oxidation number in OF2 is +2, and the oxidation number in peroxides such as H2O2 is -1 (and superoxides, the oxidation number is -1/2)

It is also convenient to know the charge on these common ions:

Nitrate is minus one: NO3-

Hydroxide is minus one: OH-

Sulfate is minus two: SO42-

Carbonate is minus two: CO32-

Phosphate is minus three: PO43-

Ammonium is plus one: NH4+

Oxidation Numbers

What is the oxidation number of element X in XF2?

Since each fluoride anion always has one net negative charge, element X must have two net positive charges to provide electrical neutrality for compound XF2.

Oxidation Numbers

What is the oxidation number of nickel?

Most often +2, but sometimes +3 in nickel compounds. The oxidation number of nickel in pure nickel is 0, as in all pure elements.

Oxidation Numbers

What is the oxidation number of N in NH4NO3?

The two nitrogen atoms are in different oxidation states. The one in the ammonium ion (NH4+) is in the 3- oxidation state while the one in the nitrate ion (NO3-) is in the 5+ oxidation state.

Oxidation Numbers

What is the oxidation number of s in PbSo4?

O= -2 and there is 4 O's so that equals -8

Pb= +2

-8 + 2 = +6

therefore S= +6

Oxidation Numbers

What is the Oxidation number in each atom in KNO3?

In KNO3 we have atoms of potassium, nitrogen and oxygen.

Since, Potassium is an alkali metal therefore, it is highly electropositve and always has an oxidation state of +1 whereas oxygen always has oxidation state of -2 except when bonded with Fluorine(which has a higher electronegativity).

We know that the molecule of KNO3 is neutral in nature i.e. it contains no overall charge on it.Therefore, sum of all oxidation states must be zero.

=> +1 +N + 3x (-2) = 0

=> +1 +N -6=0


Therefore, Oxidation state of N is +5 .

So we get, O.S. of potassium as +1, of oxygen as -2 and of Nitrogen as +5.

Oxidation Numbers

What compounds can be formed from elements X with oxidation number 3 plus and 5 plus and element Z with oxidation numbers 2 and 3?

First set. (3+,2-)


both match at 6

Second set (5+,3-)


bath match at 15

Oxidation Numbers

What is the oxidation number of C in C2H5OH?

Since H6 = +1 x6 = +6 and O= -2 . You are left with +4, therefore each C atom has -2

Oxidation Numbers

What is the oxidation number of Phosphorus in H3PO2?

Oxidation number of P in H3PO3 is +3. To calculate the oxidation number

H= 1 except when bonded to metal

O is -2 except when bonded to F

The sum of all of the oxidation number on a molecule /ion is equal to the charge so as H3PO3 is a neutral molecule:-

0 = OxP + 3OxH + 3OxO = Oxp + 3 + (-6)

Oxp = +3

H3PO3 is phosphorus acid. However when you try to make it in aqueous solution, it "tautomerises" , turning into phosphonic acid, HPO(OH)2- same formula H3PO3, but its got a P-H bond and a P=O bond. The oxidation number of P is still 3

Oxidation Numbers

What is Phosphorus oxidation number?

The oxidation number of phosphorus is +4.

Oxidation Numbers

Calculate the oxidation number for iodine in KI04?

+1 for K

-2 for each O

+7 for I

Oxidation Numbers

What is the oxidation number of phosphorus in P2O5?

The oxidation number of P2O5 is Phisphorus +5, Oxygen -2.

Oxidation Numbers

What oxidation number does an atom have if it loses two electrons when bonding?


for example, Be, Mg, Ca, Sr or any other group 2 element in a compound with a group 6 element

Oxidation Numbers

In which compound does chlorine have the highest oxidation number?

In per chloric acid HClO4, the oxidation number of chlorine is +7.

Oxidation Numbers

What is the oxidation number of Mn metal?

Since manganese is a metallic element, its oxidation number in metallic form is 0, as for any other element.

Oxidation Numbers

What are the possible oxidation numbers for magnesium and oxygen?

In compounds, magnesium has a +2 oxidation state; in most but not quite all compounds, oxygen has a -2 oxidation state. In peroxides, oxygen has a -1 oxidation state. In oxygen fluorides, oxygen has positive oxidation states.


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