A compound is defined as

• A. Two or more elements bound together with the same ratio of atoms
• B. A mixture of elements
• C. A molecule of two identical atoms only
• D. An element alone

Answer: (A)

A compound is formed when two or more elements are chemically bound in a fixed ratio of atoms. This definite combination gives a substance with properties different from its constituent elements, and you can’t change the composition without breaking bonds or creating a different compound. For example, water is built from hydrogen and oxygen in a 2:1 ratio and their atoms are chemically bonded, so it’s a compound.

The other ideas don’t fit as well: a mixture of elements is just physically combined and can vary in composition, with no chemical bonds forming a new substance; a molecule made of two identical atoms is a diatomic molecule of one element, not a compound; and an element alone is a pure substance consisting of only one type of atom, not a combination of different elements.