Question

1. what is an element? 2. what is a compound? 3. what is a molecule?

Explanation:

1. An element is a pure substance made of only one kind of atom. It cannot be broken down into simpler substances by chemical means.
2. A compound is a pure substance formed when two or more different elements chemically combine in a fixed ratio. Compounds have properties different from their constituent elements.
3. A molecule is a group of two or more atoms held together by chemical bonds. Molecules can be of elements (e.g., $O_2$) or compounds (e.g., $H_2O$).

Answer:

1. A pure - substance made of one kind of atom, not breakable by chemical means.
2. A pure substance formed by chemical combination of different elements in a fixed ratio.
3. A group of two or more atoms held by chemical bonds.