The pure substances that have properties entirely different from the atoms that make them up are compounds.

Here's why:

* Compounds are formed when two or more different elements chemically combine in a fixed ratio. The resulting compound has properties that are distinct from the individual elements it's made of.

* Elements are the simplest pure substances that cannot be broken down into simpler substances by chemical means.

Examples:

* Water (H₂O) is a compound made of hydrogen and oxygen. It's a liquid at room temperature, while hydrogen and oxygen are both gases.

* Sodium chloride (NaCl), or table salt, is a compound made of sodium and chlorine. Sodium is a reactive metal, and chlorine is a toxic gas, but together they form a stable, edible compound.

Let me know if you'd like more examples or want to explore the properties of compounds further!