A compound is a substance that can be broken down into elements by a chemical reaction. Molecules combine through chemical reactions to form polymeric substances known as polymers. Elements, molecules, and compounds exist in many forms, such as a gas, liquid, or solid as described above. An atom is the smallest particle of an element that possesses the physical and chemical properties of that element. It is the smallest particle of an element that can enter into a chemical change.
Atoms are the basic building blocks of matter.
What the atom really looks like or how the charged particles distribute themselves and move within the atom is not exactly known. Consequently, scientists resort to various models of an atom to help explain its characteristics. However, this model is far from being the most accurate. Another model of the atom, sometimes sketched in three dimensions, attempts to show the spaces within which the electrons are confined as fuzzy volumes. For our purposes, the nucleus consists of protons and neutrons.