The Concept of Molecules

If a sample of matter is taken and it is continually divided into half, then initially each half will have the same chemical and physical properties as the original. This halving process can be continued (at least theoretically) until finally the property of the half particle no longer has the same physical and chemical properties as the original. At this point the original can be defined as a molecule. This concept originally produced the first definition of a molecule; i.e., ‘a molecule is the smallest particle of a pure chemical substance that still retains its original chemical and physical properties’. Unfortunately, this definition cannot apply to salts and metals that are often composed of large arrangements of chemically bonded atoms or ions. A more modern definition of a molecule would be a stable, electrically neutral group of a least two atoms joined by strong covalent chemical bonds.

Molecules are made up of atoms and can be made up of just two dissimilar atoms (e.g. HCl-hydrochloric acid - an atom of hydrogen joined to an atom of chlorine) or two similar atoms (e.g. O₂  Oxygen – two atoms of oxygen joined together). At the other extreme a molecule may contain many thousands of different atoms (e.g. DNA). There are some exceptions; a noble gas, such as helium is, in fact, monatomic but the single atom of helium is still usually referred to as a molecule of helium.