I love the topic oxidation, it's quite interesting.
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<h2>Oxidation numbers</h2>
<p>By giving <a data-lesson="elements" data-subject="chemistry" data-tutorial="classification-of-matter" href="https://app.ulearngo.com/chemistry/classification-of-matter/elements">elements</a> an oxidation number, it is possible to keep track of whether that element is losing or gaining <a data-lesson="orbital-shapes-and-valence-electrons" data-subject="chemistry" data-tutorial="chemistry-essential-ideas" href="https://app.ulearngo.com/chemistry/chemistry-essential-ideas/orbital-shapes-and-valence-electrons">electrons</a> during a chemical reaction. The loss of electrons in one part of the <a data-lesson="what-is-a-reaction-rate" data-subject="chemistry" data-tutorial="energy-and-chemical-reactions" href="https://app.ulearngo.com/chemistry/energy-and-chemical-reactions/what-is-a-reaction-rate">reaction</a> must be balanced by a gain of electrons in another part of the reaction.</p>
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<h3>Definition: Oxidation number</h3>
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<p>Oxidation number is the charge an atom would have if it was in a compound composed of ions.</p>
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<p>There are a number of rules that you need to know about oxidation numbers, and these are listed below.</p>
<ol>
<li>
<p>A molecule consisting of only one element always has an oxidation number of zero, since it is neutral.</p>
<p>For example the oxidation number of hydrogen in \(\text{H}_{2}\) is \(\text{0}\). The oxidation number of bromine in \(\text{Br}_{2}\) is also \(\text{0}\).</p>
</li>
<li>
<p>Monatomic ions (ions with only one element or type of atom) have an oxidation number that is equal to the charge on the ion.</p>
<p>For example, the chloride ion \(\text{Cl}^{-}\) has an oxidation number of \(-\text{1}\), and the magnesium ion \(\text{Mg}^{2+}\) has an oxidation number of \(\text{+2}\).</p>
</li>
<li>
<p>In a molecule or compound, the sum of the oxidation numbers for each element in the molecule or compound will be zero.</p>
<p>For example the sum of the oxidation numbers for the elements in <a data-lesson="unique-properties-of-water" data-subject="chemistry" data-tutorial="states-of-matter" href="https://app.ulearngo.com/chemistry/states-of-matter/unique-properties-of-water">water</a> will be \(\text{0}\).</p>
</li>
<li>
<p>In a polyatomic ion the sum of the oxidation numbers is equal to the charge.</p>
<p>For example the sum of the oxidation numbers for the elements in the sulfate ion (\(\text{SO}_{4}^{2-}\)) will be \(-\text{2}\).</p>
</li>
<li>
<p>An oxygen atom usually has an oxidation number of \(-\text{2}\). One exception is in peroxides (e.g. hydrogen peroxide) when oxygen has an oxidation number of \(-\text{1}\).</p>
<p>For example oxygen in water will have an oxidation number of \(-\text{2}\) while in hydrogen peroxide (\(\text{H}_{2}\text{O}_{2}\)) it will have an oxidation number of \(-\text{1}\).</p>
</li>
<li>
<p>The oxidation number of hydrogen is often \(\text{+1}\). One exception is in the metal hydrides where the oxidation number is \(-\text{1}\).</p>
<p>For example the oxidation number of the hydrogen atom in water is \(\text{+1}\), while the oxidation number of hydrogen in lithium hydride (\(\text{LiH}\)) is \(-\text{1}\).</p>
</li>
<li>
<p>The oxidation number of fluorine is \(-\text{1}\).</p>
</li>
</ol>
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<h4>Tip:</h4>
<p>You will notice that some elements always have the same oxidation number while other elements can <a data-lesson="chemistry-104-summary-1" data-subject="chemistry" data-tutorial="physical-and-chemical-change" href="https://app.ulearngo.com/chemistry/physical-and-chemical-change/chemistry-104-summary-1">change</a> oxidation numbers depending on the compound they are in.</p>
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<h2>Example 1: Oxidation Numbers</h2>
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<h3>Question</h3>
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<p>Give the oxidation number of sulfur in a sulfate (\(\text{SO}_{4}^{2-}\)) ion</p>
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<div>
<h3>Step 1: Determine the oxidation number for each atom</h3>
<p>Oxygen will have an oxidation number of \(-\text{2}\). (Rule \(\text{5}\), this is not a peroxide.) The oxidation number of sulfur at this stage is uncertain since sulfur does not have a set oxidation number.</p>
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<div>
<h3>Step 2: Determine the oxidation number of sulfur by using the fact that the oxidation numbers of the atoms must add up to the charge on the compound</h3>
<p>In the polyatomic \(\text{SO}_{4}^{2-}\) ion, the sum of the oxidation numbers must be \(-\text{2}\) (rule \(\text{4}\)).</p>
<p>Let the oxidation number of sulfur be \(x\). We know that oxygen has an oxidation number of \(-\text{2}\) and since there are four oxygen atoms in the sulfate ion, then the sum of the oxidation numbers of these four oxygen atoms is \(-\text{8}\).</p>
<p>Putting this together gives: \begin{align*} x + (-8) &amp; = -2 \\ x &amp; = -2 + 8 \\ &amp; = +6 \end{align*} So the oxidation number of sulfur is \(\text{+6}\).</p>
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<h3>Step 3: Write down the final answer</h3>
<p>In the sulfate ion, the oxidation number of sulfur is \(\text{+6}\).</p>
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<h2>Example 2: Oxidation Numbers</h2>
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<h3>Question</h3>
<div>
<p>Give the oxidation number of both elements in ammonia (\(\text{NH}_{3}\)).</p>
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<div>
<h3>Step 1: Determine the oxidation number for each atom</h3>
<p>Hydrogen will have an oxidation number of \(\text{+1}\) (rule 6, ammonia is not a metal hydride). At this stage we do not know the oxidation number for nitrogen.</p>
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<div>
<h3>Step 2: Determine the oxidation number of nitrogen by using the fact that the oxidation numbers of the atoms must add up to the charge on the compound</h3>
<p>In the compound \(\text{NH}_{3}\), the sum of the oxidation numbers must be \(\text{0}\) (rule \(\text{3}\)).</p>
<p>Let the oxidation number of nitrogen be \(x\). We know that hydrogen has an oxidation number of \(\text{+1}\) and since there are three hydrogen atoms in the ammonia molecule, then the sum of the oxidation numbers of these three hydrogen atoms is \(\text{+3}\).</p>
<p>Putting this together gives: \begin{align*} x + (+3) &amp; = 0 \\ &amp; = -\text{3} \end{align*} So the oxidation number of nitrogen is \(-\text{3}\).</p>
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<div>
<h3>Step 3: Write the final answer</h3>
<p>Hydrogen has an oxidation number of \(\text{+1}\) and nitrogen has an oxidation number of \(-\text{3}\).</p>
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<h2>Example 3: Oxidation Numbers</h2>
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<h3>Question</h3>
<div>
<p>Give the oxidation numbers for all the atoms in sodium chloride (\(\text{NaCl}\)).</p>
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<div>
<h3>Step 1: Determine the oxidation number for each atom in the compound</h3>
<p>This is an <a data-lesson="ionic-solids" data-subject="chemistry" data-tutorial="liquids-and-solids" href="https://app.ulearngo.com/chemistry/liquids-and-solids/ionic-solids">ionic</a> compound composed of \(\text{Na}^{+}\) and \(\text{Cl}^{-}\) ions. Using rule 2 the oxidation number for the sodium ion is \(\text{+1}\) and for the chlorine ion it is \(-\text{1}\).</p>
<p>This then gives us a sum of \(\text{0}\) for the compound.</p>
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<div>
<h3>Step 2: Write the final answer</h3>
<p>The oxidation numbers for sodium is \(\text{+1}\) and for chlorine it is \(-\text{1}\).</p>
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Oxidation Numbers

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Oxidation Numbers

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