An atom is the smallest part of an element that still retains the properties of that element. There are 3 sub-atomic particles in each atom:
Particle 1. The proton. This is found in the centre of the charge and has a positive (+) charge.
Particle 2. The neutron. It is also found in the centre of the atom and has no ("neutral") charge. If this number varies in a certain element you get an isotope of the element. For example, carbon-dating uses an isotope of carbon to put an age on very old artefacts found.
Particle 3. The electron. This is found orbiting the nucleus and it has a negative charge. In a neutral atom the number of protons is the same as the number of electrons.
Two very important numbers relating to any atom are:
Atomic Number: The number of protons only in the centre of an atom.
Atomic Weight: The number of protons and neutrons in the centre of an atom.
All atoms of the same element have exactly the same weight. Here are examples of the atomic numbers and atomic weights of some elements.
| Element Name | Atomic Number | Atomic Weight |
| Hydrogen | 1 | 1.008 grams |
| Carbon | 6 | 12.011 grams |
| Chlorine | 17 | 35.457 grams |
| Lithium | 3 | 6.940 grams |
Notice that the weights are approximately double the atomic numbers. (Except for Hydrogen because it has no neutrons.) This means that the neutrons are approximately the same weight as the protons.
One example of a group of elements is Halogens.
Halogens: These elements are Fluorine, Chlorine, Bromine, Iodine, Astatine and belong in group 7 of the periodic table.
They have 7 electrons in their outer shell and need only one more to become stable like the noble gases. This number one (1) is called its valency and it is the number of electrons which an atom of that element needs to gain or lose in order to achieve noble gas structure. The halogens tend to react with atoms that need to lose one (1) electron to become stable.
For example: Sodium and Chlorine 
Sodium Chloride
Using Chemical Symbols: Na + Cl NaCl
Sodium needs to lose one electron.
Chlorine needs to gain one electron.
The result: Common Table Salt!
Attention! Although what is made (salt) is quite harmless, the ingredients that make it are not. Sodium has a tendancy to burst into flames in water and Chlorine gas has extremely poisonous fumes that cause burning of the nose, throat and, in particular, the eyes.
The Ozone Problem
Chlorine present in the upper atmosphere causes major problems and is thought to contribute to the depletion of the ozone layer. The ozone layer is a layer in the upper atmosphere that protects life on earth from dangerous ultraviolet (UV) rays from the Sun. Chloroflourocarbons (CFCs) are man made and are present in aerosol cans. They are the main source of chlorine expelled into our atmosphere. Chlorine helps destroy the ozone layer. Chlorine molecules, when exposed to UV light, split into free chlorine atoms and these react with an ozone molecule and split it apart. Free radicals are formed which are highly reactive and they in turn attack chlorine molecules and a chain reaction occurs.
Fusion and Fission
Nuclear fusion occurs when certain light nuclei (centres of the atoms) are forced together. It unleashes massive amounts of energy. This is the method by which the Sun generates heat and light. By doing so it burns through four million tonnes of energy per second. Humans have managed to mimic another high energy yield reaction thast occurs on the Sun called nuclear fission. The first atomic bomb unleashed its vast amounts of energy by making Hydrogen atoms split apart.
