Atoms are the smallest particle of an element that can exist, and can be regarded as the building blocks of everything. Atoms can combine to form molecules. Molecules are the smallest particle of either an element or a compound that can exist independently.
The atom is made up of sub-atomic particles: the proton, the neutron and the electron. The protons and neutrons are concentrated together in a tiny, enormously dense structure in the centre of the atom, called the nucleus. The electrons orbit this nucleus at a very high speed. The various elements differ from each other in the number of protons and electrons they have. For example, gold has 79 protons in its nucleus, whilst carbon has 6. The subatomic particles carry an electrical charge: the proton is positively charged, the electron is negatively charged, whilst the neutron is neutral. Atoms are electrically neutral because they contain
equal numbers of protons and electrons. The chemical properties of elements depend on the structure of their atoms. It is the arrangements of the electrons around the nucleus that give elements their particular chemical properties. Electrons are arranged in ‘shells’, and it is the state of the outermost shell which is crucial. A stable atom has a complete outer shell - only the elements known as the noble gases (such as helium) have this structure, and so they are stable as single atoms. Other elements have incomplete outer shells, so they bond with other atoms to form stable molecules.
Isotopes are atoms of an element with the same number of protons and electrons but with a different number of neutrons and therefore differing atomic masses. Isotopes are either stable or radioactive.