Which of the following best describes a compound?

A compound is best described as a chemical substance formed from two or more elements that are chemically bonded together. This definition highlights the key characteristics of compounds, which include the fact that they consist of different kinds of atoms. These atoms combine in fixed ratios and through chemical bonds, resulting in a unique substance with specific properties that are distinct from those of the individual elements that comprise it.

For example, water is a compound formed from hydrogen and oxygen in a 2:1 ratio, creating a substance with its own distinct properties, such as being a liquid at room temperature. This illustrates how compounds can exhibit different behaviors and characteristics compared to their constituent elements.

The other options do not accurately characterize a compound. A compound cannot be made up of just one element or consist only of physical properties, as these descriptions apply to elements or mixtures instead. Additionally, a mixture of gases alone does not necessarily form a compound since a mixture consists of substances that are not chemically bonded. Therefore, the definition of a compound is specifically tied to the chemical combination of different elements.