Oxidation Number Guide

[John Calonia 1D]

How do we find oxidation numbers and is it useful to make a lewis structure before attempting?

[Courtney Situ 2B]

Hello!

I would use lewis structures to find formal charges and chemical formulas to find oxidation numbers—formal charge divides bonding pairs equally, and oxidation numbers assign both the electrons in a bonding pair to the more electronegative atom in the pair. However, both should add up to the overall charge of the molecule (0 if the compound is neutral or the charge if you're calculating for a polyatomic atom). Based on the chemical formula, I follow the oxidation number rules to assign the correct oxidation number: https://www.thoughtco.com/rules-for-ass ... ers-607567.

Here's a summary of the rules:
1. Free elements (such as F2, O2, or He) have an oxidation number of 0.
2. The oxidation number of a monatomic ion = the charge of the ion (ex. Na+ is +1; N3- is -3)
3. Hydrogen in a compound is usually +1. Other than that, it is -1 only when it is part of a compound that contains elements that are less electronegative than hydrogen (such as CaH2, but I feel like this is rare).
4. Oxygen in a compound is usually -2. There are many exceptions, like if it bonds with a more electronegative atom like F (OF2, the O would be +2 instead of -2). In H2O2, the oxygen has an oxidation number of -1 instead of -2 b/c the H is +1. Other exceptions are due to structure, like BaO2, where the O bonds to itself. In this case, you should draw the lewis structure but I believe this is pretty rare.
5. The sum of the oxidation numbers of all of the atoms in a neutral compound is 0. The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion. For example, the sum of the oxidation numbers for (PO4)3- is -3.

The other rules in the article go along the lines of matching oxidation number to group/column:
The oxidation number of a Group IA element in a compound is +1.
The oxidation number of a Group IIA element in a compound is +2.
The oxidation number of a Group VIIA element in a compound is -1, except when that element is combined with one having a higher electronegativity. The oxidation number of Cl is -1 in HCl, but the oxidation number of Cl is +1 in HOCl.
Transition metals have the oxidation number represented as roman numerals in the compound name (ex. Iron (II) and Iron (III))

Hope this helps! I would do more problems for practice :)

[Alara Aygen 3K]

Hello!

I would use lewis structures to find formal charges and chemical formulas to find oxidation numbers—formal charge divides bonding pairs equally, and oxidation numbers assign both the electrons in a bonding pair to the more electronegative atom in the pair. However, both should add up to the overall charge of the molecule (0 if the compound is neutral or the charge if you're calculating for a polyatomic atom). Based on the chemical formula, I follow the oxidation number rules to assign the correct oxidation number: https://www.thoughtco.com/rules-for-ass ... ers-607567.

Here's a summary of the rules:
1. Free elements (such as F2, O2, or He) have an oxidation number of 0.
2. The oxidation number of a monatomic ion = the charge of the ion (ex. Na+ is +1; N3- is -3)
3. Hydrogen in a compound is usually +1. Other than that, it is -1 only when it is part of a compound that contains elements that are less electronegative than hydrogen (such as CaH2, but I feel like this is rare).
4. Oxygen in a compound is usually -2. There are many exceptions, like if it bonds with a more electronegative atom like F (OF2, the O would be +2 instead of -2). In H2O2, the oxygen has an oxidation number of -1 instead of -2 b/c the H is +1. Other exceptions are due to structure, like BaO2, where the O bonds to itself. In this case, you should draw the lewis structure but I believe this is pretty rare.
5. The sum of the oxidation numbers of all of the atoms in a neutral compound is 0. The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion. For example, the sum of the oxidation numbers for (PO4)3- is -3.

The other rules in the article go along the lines of matching oxidation number to group/column:
The oxidation number of a Group IA element in a compound is +1.
The oxidation number of a Group IIA element in a compound is +2.
The oxidation number of a Group VIIA element in a compound is -1, except when that element is combined with one having a higher electronegativity. The oxidation number of Cl is -1 in HCl, but the oxidation number of Cl is +1 in HOCl.
Transition metals have the oxidation number represented as roman numerals in the compound name (ex. Iron (II) and Iron (III))

Hope this helps! I would do more problems for practice :)

As someone who was also struggling with this, thank you this was super helpful!