What defines a hydrocarbon?

A hydrocarbon is defined as a compound that contains only carbon and hydrogen atoms. This definition encompasses a vast group of organic compounds, which can be either aliphatic (such as alkanes, alkenes, and alkynes) or aromatic compounds. These varied structures play a crucial role in many areas of chemistry, including fuel sources, synthetic materials, and biological systems.

The relevance of this definition is supported by the distinctive properties and behaviors of hydrocarbons. For example, alkanes are saturated hydrocarbons with single bonds and are typically less reactive than unsaturated hydrocarbons, which contain double or triple bonds. The simple structure of hydrocarbons makes them fundamental building blocks in organic chemistry.

Other choices involve elements that are not exclusively carbon and hydrogen, such as nitrogen, oxygen, or sulfur. These compositions lead to different classes of compounds—amines when nitrogen is present, alcohols or ethers with oxygen, and thiols or sulfides with sulfur—none of which fit the strict definition of hydrocarbons. Thus, the definition that aligns with the characteristics of hydrocarbons is the one that specifies only carbon and hydrogen as constituent elements.