A molecule is two or more atoms joined together, and with an overall neutral charge. (An overall positive or negative charge would make that molecule a molecular ion, or just ion for short.)

Molecules can have any number of atoms more than one.

The simplest molecules have two atoms. These two atoms can be of the same element, such as hydrogen gas (H₂) or oxygen gas (O₂). The two atoms can also be of two different elements, such as carbon monoxide (CO).

Very large molecules are called macromolecules, and proteins, nucleic acids and carbohydrates are all biological examples of these. The largest human protein, titin, has 539,022 atoms and is over one micrometre long, larger than some bacteria! Its formula is C_169,719H_270,466N_45,688O_52,238S₉₁₁.