How to Calculate Oxidation Numbers Introduction

Tyler DeWitt

4 chapters7 takeaways9 key terms5 questions

Overview

This video introduces the concept of oxidation numbers and provides a step-by-step guide on how to calculate them for elements within chemical compounds. It outlines key rules, such as elements in their elemental form having an oxidation number of 0, monatomic ions having oxidation numbers equal to their charge, and the sum of oxidation numbers in a neutral compound equaling 0. The video then demonstrates these rules through various examples, including binary compounds, compounds with oxygen and hydrogen, and polyatomic ions, highlighting common exceptions and the importance of context, like an element's position in the periodic table or its bonding partners.

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Chapters

Oxidation numbers are assigned to elements in compounds to track electron transfer.
An element by itself (in its elemental form) always has an oxidation number of 0.
Monatomic ions have an oxidation number equal to their ionic charge, with the sign written first (e.g., +2, -3).

Understanding these fundamental rules provides a baseline for calculating oxidation numbers and is crucial for interpreting chemical reactions and bonding.

Elemental sulfur (S8) has an oxidation number of 0 for sulfur because it exists as an element by itself. Potassium ion (K+) has an oxidation number of +1.

For neutral compounds, the sum of all oxidation numbers must equal 0.
Group 1A elements (like K) typically have an oxidation number of +1.
Halogens (like Cl) typically have an oxidation number of -1, unless bonded to oxygen or a more electronegative halogen.
Group 2A elements (like Mg) typically have an oxidation number of +2.

This chapter demonstrates how to apply general rules and periodic trends to solve for unknown oxidation numbers in simple compounds, reinforcing the principle of charge balance.

In KCl, Potassium (Group 1A) is +1, and Chlorine (a halogen) is -1, summing to 0. In MgO, Magnesium (Group 2A) is +2, and Oxygen is typically -2, summing to 0.

Oxygen usually has an oxidation number of -2, except in peroxides (like H2O2) where it is -1.
Hydrogen usually has an oxidation number of +1 when bonded to nonmetals.
Hydrogen has an oxidation number of -1 when bonded to metals.

These elements are common in many compounds, and their oxidation numbers follow specific patterns that are essential for accurate calculations in diverse chemical species.

In NH3, Hydrogen is bonded to Nitrogen (a nonmetal), so H is +1. Since there are 3 H atoms, the total is +3. For the compound to be neutral (sum to 0), Nitrogen must be -3. In ZnH2, Hydrogen is bonded to Zinc (a metal), so H is -1. With 2 H atoms, the total is -2. For the compound to be neutral, Zinc must be +2.

For polyatomic ions, the sum of oxidation numbers equals the ion's overall charge.
When an element's common rule conflicts with another element's rule (e.g., halogens with oxygen), the more electronegative element's rule might be overridden.
Fluorine is always assigned an oxidation number of -1 in compounds.

This section tackles more complex scenarios, including ions and compounds with exceptions, requiring a deeper understanding of electronegativity and the application of rules in sequence.

In the NO2- ion, Oxygen is -2 (total -4). To achieve the ion's charge of -1, Nitrogen must be +3. In ClF3, Fluorine is always -1 (total -3). To make the neutral compound sum to 0, Chlorine must be +3, an exception to its usual -1 rule.

Key takeaways

1Oxidation numbers are a bookkeeping tool to track electron distribution in chemical compounds.
2The sum of oxidation numbers in a neutral compound is always zero.
3The sum of oxidation numbers in a polyatomic ion equals the charge of the ion.
4Elements in their pure, uncombined form have an oxidation number of zero.
5Monatomic ions have oxidation numbers equal to their charge.
6Oxygen is typically -2, and Hydrogen is typically +1 (with nonmetals) or -1 (with metals).
7Exceptions to general rules exist, especially when elements like halogens are bonded to oxygen or fluorine.

Key terms

Oxidation NumberMonatomic IonNeutral CompoundPolyatomic IonElemental FormGroup 1A ElementsGroup 2A ElementsHalogensPeroxide

Test your understanding

1How does the oxidation number of an element in its elemental form differ from that of a monatomic ion?
2What is the rule for the sum of oxidation numbers in a neutral compound, and why is this principle important?
3Explain the typical oxidation numbers for oxygen and hydrogen and identify the conditions under which these numbers change.
4How would you determine the oxidation number of an element in a polyatomic ion?
5Describe a scenario where a halogen might have a positive oxidation number.