Final answer:
An atom is the smallest unit of matter that retains all of the chemical properties of an element. A molecule is a group of two or more atoms held together by chemical bonds. An element is a substance that consists of only one type of atom.
Step-by-step explanation:
Atom: A basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. Atoms are the building blocks of matter and the smallest particles of an element that have the properties of that element.
Molecule: Two or more atoms held together by chemical bonds to act as a unit. Molecules are the smallest part of a substance that has the physical and chemical properties of that substance.
Element: A substance containing atoms of only one type of atom. Elements are the purest form of matter and cannot be broken down into simpler substances by chemical means.