What is defined as a substance whose molecules contain atoms of different elements?

The correct answer is a compound. A compound is defined as a substance formed when two or more different elements join together through chemical bonds, resulting in a new substance with unique properties. For example, water (H2O) is a compound made up of hydrogen and oxygen atoms. When these different elements combine, they create a molecule that has characteristics distinct from the individual elements.

In contrast, an element is a pure substance that cannot be broken down into simpler substances and consists of only one type of atom. A molecule refers to two or more atoms bonded together, which can be the same or different elements; however, a molecule composed of different elements specifically classifies it as a compound. Lastly, a mixture is a combination of two or more substances that retain their individual properties and can be separated by physical means, unlike compounds that form new substances with different properties.