Understanding Combined and Pure Forms of Carbon

The question asks us to identify which among the given options is NOT a combined form of Carbon. A "combined form" of an element, in chemistry, usually refers to a compound where the element is chemically bonded with one or more other elements. A substance that is NOT a combined form is typically the element itself, either in its pure elemental state or as an allotrope.

Analyzing the Options

Let's examine each option to determine if it is a combined form of carbon:

• Chalk: Chalk is primarily composed of Calcium Carbonate. The chemical formula for Calcium Carbonate is $\text{CaCO}_3$. In this compound, Carbon ($\text{C}$) is chemically bonded with Calcium ($\text{Ca}$) and Oxygen ($\text{O}$). Therefore, Chalk is a combined form of Carbon.
• Marble: Marble is also mainly composed of Calcium Carbonate. Just like chalk, its chemical formula is $\text{CaCO}_3$. Carbon is bonded with Calcium and Oxygen. Hence, Marble is a combined form of Carbon.
• Diamond: Diamond is a well-known substance. Chemically, Diamond is made up entirely of Carbon atoms. It is one of the allotropes of Carbon, meaning it is a different structural form of the same element. Diamond consists only of Carbon atoms strongly bonded to each other in a crystal lattice structure. It is not a compound with other elements. Thus, Diamond is a pure form, not a combined form, of Carbon.
• Dolomite: Dolomite is a mineral composed of Calcium Magnesium Carbonate. Its chemical formula is $\text{CaMg(CO}_3)_2$. In Dolomite, Carbon ($\text{C}$) is chemically bonded with Calcium ($\text{Ca}$), Magnesium ($\text{Mg}$), and Oxygen ($\text{O}$). Therefore, Dolomite is a combined form of Carbon.

Conclusion

Based on the analysis:

• Chalk ($\text{CaCO}_3$) is a combined form of Carbon.
• Marble ($\text{CaCO}_3$) is a combined form of Carbon.
• Diamond ($\text{C}$) is a pure form (allotrope) of Carbon.
• Dolomite ($\text{CaMg(CO}_3)_2$) is a combined form of Carbon.

The question asks which is NOT the combined form of Carbon. From our analysis, Diamond is the only option that is not a chemical compound containing Carbon; it is elemental Carbon.

Summary Table: Forms of Carbon in Options

Therefore, Diamond is the substance that is NOT a combined form of Carbon.

Revision Table: Key Carbon Concepts

Additional Information: Allotropes of Carbon

Allotropes are different structural modifications of an element. Carbon exists in several allotropic forms due to its ability to bond in various ways. These forms have different physical properties even though they are made of the same atom.

• Diamond: Atoms arranged in a tetrahedral lattice. Extremely hard, non-conductor of electricity.
• Graphite: Atoms arranged in layers of hexagonal lattices. Soft, good conductor of electricity.
• Fullerenes: Spherical, ellipsoidal, or cylindrical structures (like Buckminsterfullerene, $\text{C}_{60}$).
• Graphene: A single layer of graphite, one atom thick, arranged in a honeycomb lattice.

Understanding the difference between an element's pure forms (allotropes) and its compounds (combined forms) is fundamental in chemistry.