What is the difference between an element, compound, and molecule?
Elements are one kind of atom. If you have a piece of sodium, every atom is sodium, each having the same number of protons.
A compound has more than one element chemically bonded. This bond is a link between the nuclei of the atoms and the electrons present.
There are three general kinds of chemical bonds : ionic bonds, covalent bonds, and metallic bonds. Depending on which bond is used to make a compound, you might have an ionic compound, a metallic compound (alloy) or a molecular compound (uses covalent bonds).
A covalent bond is when one or more pairs of electrons are SHARED by atoms that are next to each other.
A molecule is a neutral particle (no overall charge) that is held together by covalent bonds. It is also the smallest part of a molecular compound that keeps the properties (chemical and physical properties) of the compound.
In order to know if something is held together by covalent bonds, you have to know something about electronegativity but I'm not going to go that far with this answer.
So to sum it up :
element = one kind of atom
a compound is a general term for more than one element chemically bonded
a molecule is a neutral particle held together with covalent bonds