Question

In many of its compounds, oxygen has an oxidation state of \(-2 .\) However, there are exceptions. What is the oxidation state of oxygen in each of the following compounds? (a) \(\mathrm{OF}_{2} ;\) (b) \(\mathrm{O}_{2} \mathrm{F}_{2} ;\) (c) \(\mathrm{CsO}_{2}\); (d) \(\mathrm{BaO}_{2}\)

Short answer

The oxidation states of oxygen in the given compounds are: (a) +2 in \(OF_2\), (b) +1 in \(O_2F_2\), (c) -0.5 in \(CsO_2\), and (d) -1 in \(BaO_2\).

Step-by-step solution

Oxidation state in \(OF_2\)

For \(OF_2\), the oxidation state of fluorine (F) is always -1. Since the overall charge of the neutral molecule must be zero, the sum of the oxidation number of oxygen and fluorine must equal zero. Therefore, the oxidation number of oxygen is +2.

Oxidation state in \(O_2F_2\)

For \(O_2F_2\), the oxidation state of two fluorine atoms is -2 since the oxidation state of each fluorine atom is -1. To balance the charge, two oxygen atoms should have an oxidation number of +2, hence each oxygen atom will have an oxidation number (+2/2) of +1.

Oxidation state in \(CsO_2\)

For \(CsO_2\), the oxidation state of cesium (Cs) is always +1. Then since we have two oxygen atoms and the overall charge of the molecule is zero, the sum of the oxidation states of two oxygen atoms should be -1. Hence, the oxidation state of each oxygen atom is -1/2, or -0.5.

Oxidation state in \(BaO_2\)

For \(BaO_2\), the oxidation state of barium (Ba) is always +2. Because it is a neutral compound, the sum of the oxidation states of the two oxygen atoms should equal -2. Hence, the oxidation state of each oxygen atom is -1.

Key concepts

Understanding Oxygen Compounds

In chemistry, oxygen compounds are those substances that include at least one oxygen atom bonded to another element. Oxygen is highly electronegative, meaning it tends to attract electrons to itself. Generally, this results in oxygen having an oxidation state of -2. However, in some compounds, particular interactions between the atoms lead to exceptions in this typical oxidation state. Fluorine, being even more electronegative, will affect the oxidation state of oxygen when both appear in a compound. To understand these exceptions, it is important to explore different compounds such as OF₂, O₂F₂, CsO₂, and BaO₂.
These molecules illustrate how the context of the surrounding atoms, such as those with higher electronegativity, can alter the expected oxidation numbers for oxygen.

Fluorine's Unique Oxidation Effect

Fluorine is the most electronegative element, making its presence in compounds highly influential over oxidation states. In compounds like OF₂ and O₂F₂, the oxidation state of oxygen deviates from the normal -2 seen in most of its compounds. For OF₂, fluorine dictates its typical -1 oxidation state, and keeps the molecule charge-neutral by forcing oxygen into an unusual +2 oxidation state.
In O₂F₂, the shared influence of two fluorine atoms, each with an oxidation state of -1, results in oxygen being at +1 each as we distribute across two oxygen atoms. This is a great example of how powerful fluorine's electronegativity is, forcing oxygen away from its regular role in oxidation.

Neutral Molecules and Their Oxidation Balancing Act

A neutral molecule is one in which the sum of all the atoms’ oxidation numbers is equal to zero. This requirement for charge neutrality often creates interesting challenges, especially when unique oxidation states are involved, as seen in compounds like CsO₂ and BaO₂. In CsO₂, while cesium takes a standard oxidation number of +1, each oxygen ends up with an unusual -0.5 because of their partnership in the peroxide ion. Meanwhile, BaO₂ demonstrates another aspect of neutrality. Here, with barium at +2, each oxygen adopts a -1 oxidation state, making the peroxide compound charge-neutral overall.
The need for a balanced charge pushes elements to adjust their typical oxidation roles, thus creating exceptions such as these.

Exploring Chemical Compounds Through Oxidation States

Chemical compounds can vary greatly in their formation and properties. The oxidation state is a powerful tool in understanding how these compounds behave and react. Examining structures like CsO₂ and BaO₂ offers insights into the complex interactions between atoms. In chemistry, these oxidation numbers help us predict the behavior of the atoms, determine possible reactions, and deduce the molecule's stability and structure. For instance, the peculiar oxidation states in the examined compounds guide us in understanding their formation and potential uses in various applications. Therefore, mastering oxidation states becomes crucial in the broader learning of chemical compounds.