The process that occurs when two equivalent forms of a compound are in equilibrium is

Analyze the question

The question asks for the term that describes two equivalent forms of a compound existing in equilibrium. The key phrase is "equivalent forms in equilibrium."

Explain the concepts

• Isotopy: Atoms of the same element with the same number of protons but different numbers of neutrons. This is about atoms, not forms of a compound.
• Isomerism: Compounds share the same molecular formula but have different structures and properties. They are different compounds, not equivalent forms. Special cases like tautomerism involve an equilibrium between non-equivalent forms (e.g., keto-enol).
• Resonance (also called mesomerism): A molecule or ion is represented by two or more contributing structures that differ only in electron arrangement, not atom positions (e.g., benzene's two Kekulé structures, or the carbonate ion). The structures are equivalent, and the real molecule is a hybrid of them.
• Reforming: A petroleum refining process that converts straight-chain hydrocarbons into branched or aromatic ones to improve octane rating. It is unrelated to this question.

Solution

1. Isotopy relates to atoms, so eliminate A.
2. Reforming is an industrial petroleum process, so eliminate D.
3. Isomerism involves different compounds, which are not equivalent forms, so C does not fit well.
4. Resonance involves equivalent contributing structures, which is the closest match to the question's wording.

A caution: in strict chemistry, resonance structures are not actually in equilibrium. They are hypothetical representations, and the real molecule is a single hybrid.

Answer: B. Resonance

Exam tip: When you see "equivalent forms" or "delocalised electrons" (benzene, carbonate, nitrate ions), think resonance. When you see "same molecular formula, different structure," think isomerism.