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

Introduction / Context:
Allotropy is the phenomenon where an element exists in more than one structural form in the same physical state. Carbon is famous for its many allotropes, including diamond, graphite, graphene and fullerenes. Because of this variety, exam questions often ask learners to pick out which substance is not an allotrope among a list of carbon containing materials. This tests both knowledge of carbon structures and the definition of an allotrope.

Given Data / Assumptions:
- Option A is diamond, a three dimensional network solid of carbon atoms.
- Option B is graphene, a single layer of carbon atoms arranged in a hexagonal lattice.
- Option C is ethylene, a simple hydrocarbon with formula C2H4.
- Option D is buckminsterfullerene, a spherical fullerene also known as C60.
- We are to identify which one is not an allotrope of elemental carbon.

Concept / Approach:
An allotrope must consist of only one element, differing only in the arrangement of atoms. Diamond, graphene and buckminsterfullerene are all forms of pure carbon in which only carbon atoms are present and arranged differently. Ethylene, however, is a hydrocarbon containing both carbon and hydrogen atoms and is classified as a molecular compound, not an elemental form. Therefore it cannot be an allotrope of carbon, even though it contains carbon atoms in its structure.

Step-by-Step Solution:
Step 1: Recall that diamond is a pure carbon allotrope with each carbon atom sp3 hybridised and bonded to four neighbours in a three dimensional network. Step 2: Recall that graphene is a single layer of graphite like structure with sp2 hybridised carbon atoms arranged in a two dimensional honeycomb lattice. Step 3: Recall that buckminsterfullerene, C60, is a spherical molecule composed entirely of carbon atoms arranged in pentagons and hexagons, also an allotrope. Step 4: Recognise that ethylene has the formula C2H4, containing both carbon and hydrogen, so it is not a pure form of elemental carbon. Step 5: Conclude that ethylene is not an allotrope of carbon, while the other three options are.

Verification / Alternative check:
An alternative way to check is to ask whether the substance can be written with only carbon in its formula and whether it is commonly listed among carbon allotropes in textbooks. Diamond is always included as a classic allotrope. Graphene is recognised as a modern two dimensional allotrope related to graphite. Buckminsterfullerene is often cited as a fullerene allotrope. Ethylene, on the other hand, is taught in the context of organic molecules and alkenes, not as an elemental form. This comparison confirms that ethylene is the odd one out.

Why Other Options Are Wrong:
Option A, diamond, is a well known allotrope of carbon and therefore cannot be the correct answer to a question asking for a non allotrope. Option B, graphene, is a newer but firmly established carbon allotrope and is often highlighted for its unique electronic properties. Option D, buckminsterfullerene, is a recognised fullerene allotrope consisting solely of carbon atoms arranged in a spherical shell. All three are pure carbon forms and meet the definition of allotropes, so they are not correct choices here.

Common Pitfalls:
Students sometimes confuse the idea of a compound containing carbon with an allotrope of carbon, assuming that any carbon containing substance qualifies. Another pitfall is unfamiliarity with newer allotropes like graphene and fullerenes, which may lead learners to incorrectly exclude them. Remembering that an allotrope must involve only one element and that carbon forms many such pure structures helps distinguish substances like ethylene, which are organic compounds, from true allotropes.

Final Answer:
The substance that is not an allotrope of carbon is Ethylene.