An ionic compound of potassium and oxygen has the empirical formula KO. Would you expect this compound to be potassium(II) oxide or potassium peroxide? Explain.

An ionic compound is formed when metals react with non-metals, transferring electrons from the metal to the non-metal, resulting in a compound consisting of ions. Ions are charged particles that come into existence when atoms, or groups of atoms, lose or gain electrons. The metal atoms become positively charged ions (cations) because they lose electrons. Non-metals become negatively charged ions (anions) due to the gain of electrons.

The empirical formula of an ionic compound provides the lowest whole number ratio of ions within the compound. It is critical to understand that the charges must balance, meaning the total charge of cations should be equal in magnitude to the total charge of anions. In our exercise example, the empirical formula KO suggests a 1:1 ratio of potassium to oxygen. If considered as an ionic compound, this ratio does not seem to correctly demonstrate the charge balance since potassium usually forms a +1 cation, whereas oxygen typically forms a -2 anion. This mismatch raises questions about the stability and correctness of the given empirical formula.

The oxidation state, or oxidation number, indicates the degree of oxidation of an atom in a substance. It reflects the number of electrons that an atom uses to bond with atoms of other elements. The oxidation states are essential for determining formulas of compounds, especially in compounds containing elements that can exhibit more than one oxidation state.

In the case of the ionic compound that contains potassium and oxygen, the standard oxidation state of potassium as a Group 1 element is +1, reflecting its tendency to lose one electron and form a K+ ion. Oxygen, usually found with an oxidation state of -2 when it forms O²⁻ ions, can also be part of special groups, such as peroxide (O₂²⁻), where each oxygen atom technically has an oxidation state of -1. An exercise that correctly identifies oxidation states can help predict the chemical formula of compounds and understand their chemical behavior. However, as shown in the solution, neither potassium(II) oxide with an expected oxidation number of +2 for potassium nor potassium peroxide with a -1 oxidation state per oxygen atom in the peroxide ion, corresponds to the empirical formula KO.

Potassium peroxide is an example of a compound where the common oxidation states deviate from the norm. Comprised of potassium (K) and the peroxide ion (O₂²⁻), it illustrates how oxygen can have an oxidation state different from the usual -2. In potassium peroxide, each oxygen atom in the O₂²⁻ ion has an oxidation state of -1. Potassium peroxide has a chemical formula of K₂O₂, indicating a 2:1 potassium to peroxide ion ratio.

The peroxide ion is larger and contains more oxygen than the simple oxide ion O²⁻, accounting for the different empirical formula compared to typical oxides. Therefore, the empirical formula KO, with an implied 1:1 ratio of potassium to oxygen, does not accurately represent potassium peroxide. When trying to determine the identity of a compound from its empirical formula, recognizing distinctive groupings of elements like the peroxide ion, and understanding their impact on stoichiometry and oxidation states, is essential.