Oxidation Number of Phosphorous Atom in Phosphorus Acid (H3PO3)
Phosphorus acid, is an inorganic weak acid has one phosphorus atom, three oxygen atoms and three hydrogen atoms. In this tutorial, we are going to learn how to find oxidation numbers of phosphorus atom in H3PO3 molecule.
There are two methods to determine oxidation numbers of atoms in molecules or ions.
- Using lewis structure of respective molecule or ion
- Using algebraic equation
Find oxidation numbers of atoms of H3PO3 molecule using the lewis structure
First, lewis structure of H3PO3 molecule should be drawn and the drawn structure is shown below.
Mark direction of electrons attracting of bonds due to electronegativity difference of atoms
Atoms which have higher electronegativity values, can attract electrons of bonds towards them. Now, we will see how this will affect to the H3PO3 molecule.
Electronegativity values of nitrogen, oxygen and hydrogen atoms - Pauling Scale
- Phosphorus: 2.19
- Oxygen: 3.44
- Hydrogen: 2.20
Now, we can look how electrons are attracted between phosphorus, oxygen and hydrogen atoms.
- Among phosphorus, oxygen and hydrogen atoms Oxygen is the most electronegative element. Due to higher electronegativity, electrons of P-O and O-H bonds are attracted towards oxygen atoms as following image.
- Considering P-H bond, you may think electrons of P-H bond attracts towards the hydrogen atom because electronegativity of hydrogen is just over phosphorous. But, we have a specific theory; when hydrogen make bonds with non-metallic atoms, oxidation number of hydrogen is considered as +1 So, electrons of P-H bond attracts towards phosphorus atom.
Oxidation number of phosphorus atom in H3PO3
- Oxidation number of phosphorus atom: According to the lewis structure of H3PO3, electrons of the two P-O bonds and one P=O bonds tend to attract towards oxygen atom. Therefore, phosphorous atom partially loses four electrons (+4 oxidation state).
- Because electrons in P-H bond are attracted towards phosphorous atom, phosphorous atom partially receives an additional electron. Due to that, phosphorous gain a -1 oxidation state.
- So oxidation number of phosphorous atom = +4 + (-1) = +3
Oxidation number of oxygen atoms in H3PO3
Each oxygen atom's oxidation number is -2.
Find oxidation numbers of atoms of H3PO3 using algebraic equation
This method use an mathematical algebraic equation to find the oxidation number of an atom in molecules or ions. In this method, you can find the unknown oxidation number of an atom by using known oxidation numbers of other atoms in the molecule or ion.
This method is not 100% accurate for all molecules and ions, but can be used in lot of applications and we will try it now.
Oxidation numbers of oxygen
Oxygen has several oxidation numbers such as -2,-1, 0, +2. Because, oxygen is the most electronegative element, oxidation number of oxygen should be a negative one. As well, because oxygen's most common oxidation number is -2, we can substitute -2 in our following algebraic equation.
Oxidation number of hydrogen
Because, when hydrogen combines with non-metallic elements, oxidation number of hydrogen is considered is +1.
Summation of individual oxidation numbers of each atom = overall charge of molecule or ion
For H3PO3 molecule, overall charge is zero and we can write following algebraic equation.
Oxidation number of phosphorous atom + oxidation number of oxygen atoms + oxygen number of hydrogen atom = overall charge of H3PO3 molecule
Because oxidation number of nitrogen atom is taken as unknown, take it as x.
Apply known values for the equation.
- x*1 + (-2)*3 + (+1)*3 = 0
- x = +3
We have found that, oxidation number of phosphorous atom in H3PO3 molecule is +3.
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