There is a tendency to consider the molecule of modern science as identical with the atom of the old philosophers but the modern atomic theory has given the molecule a different status from that of the old-time atom. Atom, as used in natural science, has a specific meaning based upon the theory of chemistry. This meaning is modified by recent work in the field of radioactivity, but the following will serve as a definition. It is the smallest partlcle of an element which can exist in a compound. An atom cannot exist alone as such.
Atoms combine with each other to form molecules.
The molecule is the smallest particle of matter which can exist without losing its distinctive properties. It corresponds pretty closely to the old Epicurean atom. The modern atom is an entirely new conception. Chemistry teaches that the thousands of forms of matter upon the earth, almost infinite in variety, can be resolved into eighty substances, unalterable by chemical processes and possessing definite spectra. These, substances are called elements.