Name: assigning oxidation states in redox reactions
Uploaded: 2020-09-03T13:38:26.000Z
Duration: 3 min 14 s
Channel: JoVE

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Physical Science

Matter and its Interactions

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Oxidation Numbers

Redox reactions between metals and nonmetals typically involve a complete transfer of electrons to form ionic compounds; hence, they are easy to identify.&nbsp;However, redox reactions involving only nonmetals with a partial transfer of electrons are not as easily identifiable.&nbsp;

Redox reactions are characterized by changes in the oxidation states of the atoms, which indicates electron movement between the atoms.

The oxidation state, or oxidation number, of an atom in a compound is the charge it would have if the shared electrons in each heteronuclear bond were completely transferred to the more electronegative atom. Homonuclear bonds are divided equally.

For instance, in gaseous hydrogen chloride, chlorine is more electronegative. If hydrogen&rsquo;s electron is transferred completely to chlorine, chlorine gets a 1&minus; charge, corresponding to the &minus;1 oxidation state, and hydrogen gets a 1+ charge, corresponding to the +1 oxidation state.

Oxidation states can be assigned to atoms in elemental form and in most ions and compounds using specific rules. The first three rules are always followed. The remaining rules are applied one by one until the first three rules are satisfied.&nbsp;

These rules will no

oxidation numbers

In redox reactions, the transfer of electrons occurs between reacting species. Electron transfer is described by a hypothetical number called the oxidation number (or oxidation state). It represents the effective charge of an atom or element, which is assigned using a set of rules.

<h4>Oxidation Number (Oxidation State)</h4>

In the case of an ionic compound, oxidation numbers are assigned based on the number of electrons transferred between reacting species. For example, in the formation of calcium chloride (CaCl2), calcium loses two valence electrons, and the two chlorine atoms gain one electron each. In CaCl2, calcium&rsquo;s oxidation state is +2, and each chlorine&rsquo;s oxidation state is &minus;1.&nbsp;

In the case of covalent compounds, electrons are not gained or lost but instead are shared between the atoms. The atom with a greater attraction for electrons pulls the shared pair more strongly. Reactions involving covalent compounds are identified as redox by applying the concept of oxidation number to track electron movements. Oxidation states help us easily identify the species being oxidized and reduced in redox reactions. &nbsp;

<h4>The Rules for Assigning Oxidation Number</h4>

Oxidation numbers can be positive, negative, or zero. They are assigned based on the following rules:

<ol>
	<li>All free elements have an oxidation number zero. The elements could be monoatomic, diatomic, or polyatomic.&nbsp;</li>
	<li>In a compound, group 1A elements (all alkali metals) have an oxidation number of +1, while group 2A elements (all alkaline earth metals) have an oxidation number of +2.&nbsp;</li>
	<li>Halogens usually have an oxidation number of &minus;1, except in their compounds with oxygen, where they have a positive oxidation state.&nbsp; 
	Fluorine is the most electronegative element. It has a &minus;1 oxidation state in all its compounds.</li>
	<li>For monoatomic ions, the oxidation number is the same as the charge on the ion.&nbsp;</li>
	<li>Oxygen always has an oxidation number of &minus;2, except in peroxides, where its oxidation number is &minus;1.</li>
	<li>Hydrogen has an oxidation state of +1 with nonmetals and &minus;1 with metals.</li>
	<li>The sum of the oxidation number for a neutral compound is zero, while for a polyatomic ion, it is equal to the charge on the ion.</li>
</ol>

This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions">Openstax, Chemistry 2e, Section 4.2: Classifying Chemical Reactions.</a>

In redox reactions, the transfer of electrons occurs between reacting species. Electron transfer is described by a hypothetical number called the ...

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KEY TERMS AND CONCEPTS

assigning oxidation states in redox reactions

Oxidation states help track electron transfer in redox reactions. An oxidation number, also called an oxidation state, is a hypothetical number that shows the effective charge of an atom or element. Chemists assign it by using a set of rules.

In an ionic compound, oxidation numbers are based on how many electrons move between reacting species. For example, when calcium chloride forms, calcium loses two valence electrons and the two chlorine atoms each gain one electron. In CaCl2, calcium has an oxidation state of +2, and each chlorine has an oxidation state of -1.

Covalent compounds work differently because electrons are shared instead of fully transferred. The atom with the stronger attraction for electrons pulls the shared pair more strongly. Oxidation numbers make it possible to track these electron shifts and identify which species is oxidized and which is reduced in a redox reaction.

Oxidation numbers can be positive, negative, or zero. Free elements have an oxidation number of zero, whether they are monoatomic, diatomic, or polyatomic. In compounds, group 1A elements have an oxidation number of +1, and group 2A elements have an oxidation number of +2. Halogens usually have an oxidation number of -1, but they can be positive in compounds with oxygen. Fluorine is the most electronegative element, and it has an oxidation state of -1 in all its compounds.

For monoatomic ions, the oxidation number matches the ion charge. Oxygen usually has an oxidation number of -2, except in peroxides, where it is -1. Hydrogen has an oxidation state of +1 with nonmetals and -1 with metals. The oxidation numbers in a neutral compound add up to zero, while the total for a polyatomic ion equals the ion charge.

Assigning Oxidation States in Redox Reactions

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