An atom represents the smallest part of an element that still retains the characteristics of that element, and the bonding between two or more atoms forms a molecule. Molecules are often defined as the smallest part of a chemical compound formed from the bonding of atoms of different elements, but a molecule can also consist of a bond between the same type of atom, such as two hydrogen atoms bonding to form a hydrogen (H2) molecule. In their strictest definition, the molecules of compounds contain two or more atoms of different elements and possess neither a negative nor a positive net electrostatic charge, and those molecules that consist of the atoms of only one type of element, such as H2 or O2, are called homonuclear molecules.
The term "molecule" is used less strictly outside of chemistry. For example, the single non-bonded atoms of a noble gas may be referred to as "molecules" in kinetic gas theory. The molecules of compounds that possess a negative or a positive charge are, according to the chemical definition, called ions, but may be referred to as "molecules" in organic chemistry or quantum physics.
Molecules can be broken down into their individual atoms, and the individual atoms can be further divided into their three major sub-atomic particles: electrons, protons and neutrons. At this level, the particles no longer exhibit any of the characteristics of the individual chemical element they come from. An atom's protons and neutrons are also called nucleons and are found in the atom's nucleus, the portion of an atom that contains the greater part of its mass. The tightly-packed nucleus, minus the electrons orbiting around it, represents the vast majority of the atom's size and is about 10,000 to 50,000 times smaller than the size of the entire atom.