Rules for oxidation numbers

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Ashley Kenney 1E
Posts: 32
Joined: Wed Nov 14, 2018 12:23 am

Rules for oxidation numbers

Postby Ashley Kenney 1E » Sat Feb 23, 2019 11:56 pm

What are the rules that we should know for assigning oxidation numbers? Thanks!

Matthew Tran 1H
Posts: 165
Joined: Fri Sep 28, 2018 12:16 am

Re: Rules for oxidation numbers

Postby Matthew Tran 1H » Sun Feb 24, 2019 12:24 am

I would memorize the following:

1. Any element by itself (no charge) has an oxidation number of 0 (H2, Cl2, O2, O3, etc.)
2. H typically has an oxidation number of +1
3. O typically as an oxidation number of -2 (except in H2O2 where it's -1)
4. Halogens (Cl, F, Br, I) typically have an oxidation number of -1
5. Alkali metals (Group 1) typically have an oxidation number of +1
6. Alkaline earth metals (Group 2) typically have an oxidation number of +2

These oxidation numbers should be fairly intuitive by looking at the periodic table. Using these you should be able to calculate the other oxidation numbers for elements in compounds by making sure that the sum of the oxidation numbers multiplied by the number of atoms (e.g. in MnO4- there are 4 Os so 4*(-2)) equals the overall charge.

Ashe Chen 2C
Posts: 31
Joined: Mon Jan 07, 2019 8:23 am

Re: Rules for oxidation numbers

Postby Ashe Chen 2C » Sun Feb 24, 2019 1:53 am

1) The oxidation number for an atom in its elemental form is always zero.
2) The oxidation number of a monoatomic ion = charge of the monatomic ion.
3) The sum of all oxidation numbers in a neutral compound is zero. The sum of all oxidation numbers in a polyatomic ion is equal to the charge on the ion.
4) The oxidation number of an alkali metal (IA family) in a compound is +1; the oxidation number of an alkaline earth metal (IIA family) in a compound is +2.
5) The oxidation number of oxygen in a compound is usually –2. If, however, the oxygen is in a class of compounds called peroxides (for example, hydrogen peroxide), then the oxygen has an oxidation number of –1. If the oxygen is bonded to fluorine, the number is +1.
6) The oxidation state of hydrogen in a compound is usually +1. If the hydrogen is part of a binary metal hydride (compound of hydrogen and some metal), then the oxidation state of hydrogen is –1.
7) The oxidation number of fluorine is always –1. Chlorine, bromine, and iodine usually have an oxidation number of –1, unless they’re in combination with an oxygen or fluorine.


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