The atom is the smallest subdivision of an element which still retains the properties of that element. In fact, a very good definition of an atom is: the smallest part of an element that can enter into a chemical combination. There are around 118 elements known to man, of which 20-30 are really, really important.he parallel definition (to the element one above) for the molecule is: the smallest part of a compound that can enter into a chemical combination. Another definition, equally good, is that a molecule is the smallest stable part of a compound. Water is a typical example of a compound.