An atom is the smallest "piece" of an element that still has the properties of that element. For example, an atom of gold is the smallest piece of gold that still acts like gold — divide that atom any further and the particles inside are no longer gold. Atoms are much too small to see or feel with our human senses, but evidence from scientific instruments and studies clearly indicates that they are at the heart of all objects and matter in the universe. When many atoms come together, atoms tend to form molecules.

A molecule is a combination of two or more atoms bonded together. For example, a molecule of water (designated by the symbol H₂O) consists of two hydrogen atoms and an oxygen atom which are in a "relationship" — held together by an electric attraction.