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