By Chris Deziel | Updated Aug 30, 2022
Since John Dalton first proposed the existence of atoms in the early 19th century, they have remained the fundamental building blocks of matter. Modern research shows that atoms are themselves made of protons, neutrons and electrons, which are in turn composed of even smaller particles. Yet, on the constructive side, atoms assemble into countless chemical compounds that form everything around us.
In chemistry, a collection of two or more atoms held together by chemical bonds is called a molecule. Simple examples include diatomic oxygen (O₂), which you inhale each breath, and far larger examples like the titin protein in human muscle. Titin contains roughly 539,030 atoms, making it one of the largest naturally occurring molecules.
Another term you’ll encounter is compound. A compound is a molecule that contains at least two different elements arranged in a fixed ratio. All compounds are molecules, but not all molecules are compounds. Classic examples are sodium chloride (NaCl) – table salt – and water (H₂O).
Atoms are electrically neutral because they have an equal number of negatively charged electrons and positively charged protons. Electrons occupy discrete shells around the nucleus. When an atom’s outer shell is incomplete, it seeks stability by either donating or sharing electrons with neighboring atoms.
If one atom donates an electron, both atoms become charged ions with opposite charges, and they are held together by electrostatic attraction: an ionic bond. If atoms share electrons, they form a covalent bond. Covalent bonds are more common in organic chemistry and can be single, double, or triple depending on the number of shared electron pairs.
Molecules can be classified as homonuclear (made of a single element, e.g., H₂, O₂, P₄) or heteronuclear (containing two or more elements, e.g., CO₂, HCl, CH₄). Most naturally occurring molecules fall into the heteronuclear category because they are compounds.
While many molecules are electrically neutral, ionic molecules often carry a net charge and can form ionic bonds with other ions or polar molecules. Some molecules, like water, are polar—the unequal sharing of electrons creates a partial positive charge on one side and a partial negative charge on the other. This polarity underlies many unique properties of water, such as its high surface tension and ability to dissolve a wide range of substances.
