An atom is defined as a basic unit of matter that contains a centralized dense nucleus which is surrounded by an electron cloud. When two or more atoms are held together by a bond, this new entity is known as a molecule. The word "molecule" is a loose term, and it colloquially carries different meanings across different fields of study. For example, the term "molecules" is mentioned in the kinetic theory of gases, referred to as any gaseous particle regardless of its composition. Most often, the term "molecules" refers to multiple atoms; a molecule may be composed of a single chemical element, as with oxygen (O2), or of multiple elements, such as water (H2O). Atoms and complexes that are connected by non-covalent bonds are generally not considered single molecules. Molecules are neutral and carry no charge; this property distinguishes them from ions.
Molecular size varies depending on the number of atoms that make up the molecule. Most molecules are too small to be seen with the naked eye. The smallest molecule is diatomic hydrogen (H2), with a bond length of 0.74 angstroms. Because of their small size, molecules cannot be observed by the naked eye. Some small molecules may be observed by specialized microscopes. Macromolecules are large molecules composed of smaller subunits; this term from biochemistry refers to nucleic acids, proteins, carbohydrates, and lipids.
Often, a compound's composition can also be denoted by an empirical formula, which is the simplest integer ratio of its constituent chemical elements, but this empirical formula does not always describe the specific molecule in question since it provides only the ratio of its constituent elements. The elemental composition of a molecule can be exactly represented by its molecular formula, which provides the exact number of atoms that are in the molecule.
Molecules have fixed equilibrium bond lengths and bond angles about which they continuously oscillate through vibrational and rotational motions. The molecular geometry and composition determine the reactivity and other properties of a molecule. Isomers are molecules with the same atoms in different geometric arrangements; because of these different arrangements, isomers often have very different chemical and physical properties.