Atom
An atom is the basic unit of matter, comprised of protons and neutrons in its center, and with electrons on the outside. Atoms range from 0.1 to 0.5 nanometers in width[1], with one nanometer about 100,000 times smaller than the width of a human hair[2].
The type of atom is defined by the number of protons it contains, called its atomic number, such as all carbon atoms having 6 protons and all nitrogen atoms having 7 protons. Standard atoms have an equal number of protons, neutrons, and electrons. If an atom gains or loses electrons the atom is referred to as an ion. If an atom gains or loses neutrons the atom is referred to as an isotope.
Substances comprised of only one type of atom is called an element, such as 24-karat gold. However, most atoms bond with other atoms to form a molecule. If a molecule is comprised of different types of atoms it is called a compound, such as two hydrogen atoms bonding with one oxygen atom to create a water molecule, or two hydrogen atoms bonding with two oxygen atoms to create a hydrogen peroxide molecule.