When atoms of different elements combine, the molecule is called a compound.

Here's why:

* Molecule: A molecule is formed when two or more atoms bond together. These atoms can be of the same element (like O₂ for oxygen gas) or different elements (like H₂O for water).

* Compound: A compound is a specific type of molecule where the atoms are from *different* elements. This means they have different properties than the individual elements that make them up.

Example:

* Water (H₂O) is a compound because it's made of hydrogen (H) and oxygen (O) atoms.

* Oxygen gas (O₂) is a molecule, but not a compound because it's made of only oxygen atoms.