How to Calculate the Oxidation Number of Atoms in a Molecule/Ion
Chemists have developed a method to find which atoms have gained/lost electrons, especially since some reactions can seem very complicated. To determine if electrons were gained or lost by an atom, we assign an oxidation number to each atom in a compound.
This oxidation number is, put simply, the “charge” on the atom – although we also assign oxidation numbers for covalent compounds, which don't really have charged atoms inside of them.
There are some basic rules:
This oxidation number is, put simply, the “charge” on the atom – although we also assign oxidation numbers for covalent compounds, which don't really have charged atoms inside of them.
There are some basic rules:
- If an atom is in elemental form, its ON = 0
- Hydrogen is always +1, except in hydrides, where it's -1
- Oxygen is always -2, except in peroxides, where it's -1
- All other ions get whatever charge they prefer, as long as the sum of all atoms' oxidation numbers is the charge on the particle.
Examples
a) What are the oxidation numbers of the atoms in Fe2O3?
Rule 1 does not apply. Rule 2 does not apply. Rule 3 applies: each O gets a charge of -2 (this is not a peroxide). There are three oxygens, so together they contribute -6 to the particle's “charge”. So, the irons need a charge of +3 each, so that the sum of all the oxidation numbers is 0 (Rule 4)
b) What are the oxidation numbers of the atoms in the ClO3- ion?
Rule 1 does not apply. Rule 2 does not apply. Rule 3 applies: each O gets a charge of -2 (this is not a peroxide). There are 3 oxygens, so together they contribute -6 to the particle's “charge”. The total charge on the ion needs to be just -1, so the chlorine itself must have a charge of +5 (Rule 4)
c) What are the oxidation numbers of the atoms in NaH2PO4?
Rule 1 does not apply. Rule 2 applies: each H get a charge of +1. Rule 4 applies: each O gets a charge of -2 (this is not a peroxide). There are 4 such oxygen atoms. So, together, we have assigned +1 for each H (+2 total) and -2 for each oxygen (-8 total). Na can only have a charge of +1, but P can have many different valences. So, it is likely that Na gets a charge of +1 and P gets whatever charge is necessary to give the molecule a “0” total charge. One Na: +1. Two H: +2. Four O: -8. One P: +5.
Rule 1 does not apply. Rule 2 does not apply. Rule 3 applies: each O gets a charge of -2 (this is not a peroxide). There are three oxygens, so together they contribute -6 to the particle's “charge”. So, the irons need a charge of +3 each, so that the sum of all the oxidation numbers is 0 (Rule 4)
b) What are the oxidation numbers of the atoms in the ClO3- ion?
Rule 1 does not apply. Rule 2 does not apply. Rule 3 applies: each O gets a charge of -2 (this is not a peroxide). There are 3 oxygens, so together they contribute -6 to the particle's “charge”. The total charge on the ion needs to be just -1, so the chlorine itself must have a charge of +5 (Rule 4)
c) What are the oxidation numbers of the atoms in NaH2PO4?
Rule 1 does not apply. Rule 2 applies: each H get a charge of +1. Rule 4 applies: each O gets a charge of -2 (this is not a peroxide). There are 4 such oxygen atoms. So, together, we have assigned +1 for each H (+2 total) and -2 for each oxygen (-8 total). Na can only have a charge of +1, but P can have many different valences. So, it is likely that Na gets a charge of +1 and P gets whatever charge is necessary to give the molecule a “0” total charge. One Na: +1. Two H: +2. Four O: -8. One P: +5.
