The Periodic Table

This chart is known as the Periodic Table because elements had similar properties at regular intervals

In the Periodic Table, each element is identified by a symbol which relates to its name in a particular language 

Above the symbol is the atomic number of the element and below is the mass number of the element

When protons, neutrons and electrons were discovered, the elements were arranged in order of atomic number instead of relative atomic mass as Newlands and Mendeleev both decided to do. 
This principle was proposed by Henry Moseley, a British scientist, in 1913. 
This eliminated Mendeleev's problem of elements not being placed in the correct row (according to their properties) 
For example, Tellurium should have been placed after Iodine, as Tellurium has a higher relative atomic mass than Iodine. However, in order for Iodine to be placed in the same group as elements with similar properties (such as bromine, chlorine and fluorine), it had to be placed after Tellurium. 
By organising the elements in order of their atomic number, Iodine could be placed in the correct group without breaking the rules (as Iodine has a higher atomic number than Tellurium) 

In the Periodic Table, the rows are referred to as periods
                                    the columns are referred to as groups

All of the elements in a particular period have the same number of energy shells (atomic orbitals)
For example, all of the elements in Period 2 have 2 energy shells

All of the elements in a particular group have the same number of electrons in the outermost energy shell
For example, all of the elements in Group 6 have 6 electrons in their outermost shell
These outer electrons are known as valence electrons because these are the ones that are used to form chemical bonds

In the Periodic Table, there are three types of elements - Metals, Non-Metals and Metalloids
However, Metalloids are commonly classed as Non-Metals

PICTURE

The Periodic Table shows the reactivity trend of the elements:
Metals
Period
Reactivity decreases from left to right (there are less electrons that need to be lost to obtain a full outer shell)
Group
Reactivity increases from the top to the bottom (there are more energy shells so the electrons are further away from the nucleus which results in weaker bonds which causes the atom to be more reactive)
Non-Metals
Period 
Reactivity increases from left to right (there are more electrons that need to be lost to obtain a full outer shell)
Group
Reactivity decreases from the top to the bottom (there are more energy shells so the shielding effect [decrease in attraction between an electron and a nucleus which prevents the atom from gaining an electron] is greater which causes the atom to be less reactive)

Elements in Group 1 are known as Alkali Metals
Elements in Group 2 are known as Alkali Earth Metals
Elements in the block between Group 2 and Group 3 are Transition Metals
Elements in Group 7 are known as Halogens
Elements in Group 8 are known as Noble Gases

N.B - 1/08/2012
I am currently editing a copy of the Periodic Table to show the different sections (metals & non-metals and groups). This should be completed later today