The Periodic Table arranges the elements in order of proton number. Hydrogen has a proton number of 1 so heads the table, followed by helium with proton number 2. Elements whose atoms have 1 electron in their outer shell are put into Group 1 of the periodic table. Group 2 elements have 2 electrons in their outer shell and so on. The groups are arranged side by side in the periodic table. These horizontal rows are called periods. Period 1 elements have their outer electrons in the first energy shell. Period 2 elements have their outer electrons in the second energy shell. The periodic table has eight groups of elements, plus a block of transition metals. Some of the groups have special names: Group 1: Alkaline metals. Group 2: Alkaline earth metals. Group 7: Halogens. Group 0: Noble gases. The noble gases are colourless, unreactive gases. As you go down group 0 their density increases. The halogens have similar properties since they all have 7 electrons in their outer shell. Reactivity does decrease down the group as it becomes more difficult to gain an extra electron. Halogens exist as molecules. The alkali metals all have one electron in their outer shell. They are reactive soft metals. The alkali metals are reactive with water producing alkaline solutions. Reactivity increases down the group, as the loss of an electron becomes easier as the size of the atom increases. The alkaline earth metals are less reactive than group 1 since they need to lose 2 electrons to gain a full outer shell. Group 2 reactivity and melting/boiling points increase down the group, as does group1. Transition metals and their compounds are often used in industry as catalysts.