Before we start to discuss more complicated areas of chemistry, it is necessary to ensure that certain concepts of atomic structure are understood.

Firstly, the atom is composed of a nucleus (about 10^-15m in diameter) at the centre which contains most of the mass of the atom, and orbiting electrons (negatively charged particles), which have negligible mass.  The nucleus is composed of protons (positively charged fundamental particles), and neutrons (uncharged fundamental particles).  The radius of an atom is roughly 10,000 times larger than the diameter of the nucleus, i.e. it is about 10^-10m, or 1 angstrom (. 

These figures illustrate the fact that practically all of the mass of the atom is contained within a very small region.

Atoms can be described by two numbers: the atomic number (Z), which is equal to the number of protons the atom contains, and the mass number (A), which is equal to the number of protons plus neutrons. 

In a neutral atom, the number of electrons will always be equal to the number of protons.

Atoms that have identical Z numbers are atoms of the same element (e.g. Z=1 is hydrogen, Z=2 is helium etc.)  However, atoms of the same element can have different mass numbers.  This means that they have different numbers of neutrons, and such atoms are called isotopes of an element.  For example, "normal" hydrogen has Z=1, and A=1 (i.e. one proton, and no neutrons), but deuterium is an isotope of hydrogen which has Z=1, and A=2 (one proton and one neutron).

The atomic weight is the average mass of a large number of atoms of a particular element.  This must therefore take into account the relative abundances of each isotope.  Now we have discussed some of the general properties of the atom, let us take a look at the electrons, as it is they which, as chemists, interests us the most.