Which of the following substances is not an example of an allotrope of an element?

Introduction / Context:
This question examines the concept of allotropy, where the same element exists in different structural forms in the same physical state. Examples include different forms of carbon and oxygen. The task is to identify which option is not a pure allotrope but instead something else, such as a compound or a mixture. Understanding this distinction is important both in inorganic chemistry and in materials science.

Given Data / Assumptions:

• Diamond, graphite, ozone, and steel are listed.
• Allotropes are different forms of the same element in the same state of matter.
• We assume standard definitions from school chemistry textbooks.

Concept / Approach:
Carbon has several allotropes, including diamond, graphite, and amorphous varieties. Oxygen has O2 and O3 (ozone) as two allotropic forms in the gaseous state. Steel, however, is not a pure element but an alloy, mainly of iron with carbon and other metals. Therefore, any option that refers to an alloy or mixture cannot be an allotrope. We identify which of the four options represents a mixture rather than a pure elemental form.

Step-by-Step Solution:
Step 1: Examine diamond. Diamond is a crystalline form of pure carbon, an allotrope of carbon. Step 2: Examine graphite. Graphite is another crystalline form of pure carbon with layered structure, also an allotrope of carbon. Step 3: Examine ozone. Ozone is a molecular form of oxygen, O3, whereas ordinary oxygen is O2. These are allotropes of oxygen. Step 4: Examine steel. Steel is an alloy mainly composed of iron plus carbon and other elements. It is not a pure form of a single element. Thus, steel is not an allotrope.

Verification / Alternative Check:
Textbooks discussing allotropy list diamond and graphite as the most famous examples for carbon and mention ozone as an allotrope of oxygen. None of them lists steel in the context of allotropy. Instead, steel appears under alloys and metallurgy. This separation across topics confirms that steel is not considered an allotrope.

Why Other Options Are Wrong:
Option A: Diamond is a pure form of carbon with each carbon atom tetrahedrally bonded to four others, a classic allotrope. Option B: Graphite is another pure carbon form with layered planar structures, also an allotrope of carbon. Option C: Ozone is an allotropic form of oxygen, distinct from dioxygen O2 in structure and properties.

Common Pitfalls:
Some learners confuse alloys with allotropes because both words start with similar letters, or they think that any special form of a metal is an allotrope. Remember that allotropes must be different structural forms of the same pure element, while alloys are mixtures of metals and sometimes non metals. Keeping these definitions clearly separated will prevent such mix ups.

Final Answer:
The substance that is not an allotrope of an element is Steel.