Elements react to make compounds.
When elements react they do so to attain a full outer shell of electrons. In doing this they become more stable.
Now for an element to get a full outer shell of electrons means it will acquire the electron arrangement of the nearest noble gas in The Periodic Table. Noble gases typically have eight electrons in their outer shell, although helium only has two. The outer shell of helium is too small to hold eight electrons.
A column of elements in The Periodic Table is called a Group and elements within the same group have the same number of outer electrons.
So why is The Periodic Table shaped like it is?
The Periodic Table is shaped like it is because it is a great big sign post giving directions. However, it is not giving directions to different places, but to different types of chemical behaviour. What the sign is telling you is that whatever the chemical properties of a given column of elements are, they are likely to be similar for all of the elements in that group.
For example, all of the elements in group 1 of The Periodic Table have one electron in their outer shell and they all show variations in the same chemical reactions. For instance, they all react vigorously with water to produce the metal hydroxide and hydrogen gas. The metal hydroxides are all soluble in water, resulting in an alkaline solution and this group of metals is called the alkali metals. Their reactions are all similar and get more and more violent as you go down the group.
Not all of the elements in every group have the same chemical properties, especially not those in groups 3, 4 and 5, but the generalisation holds for many of the groups and is one of the central themes of the study of chemistry, looking for patterns in chemical behaviour and trying to explain them.
So why Periodic Table?
A row of elements in The Periodic Table is called a Period. The first period is at the top of the table and contains just two elements, hydrogen and helium. The second period is the next row down consisting of the elements lithium to neon, it contains eight elements in all. Period three is from sodium to argon and so on.
As one goes across each period from left to right, one observes a repeating pattern in the way the chemical properties are changing. This must be so, because the chemical properties of the elements within any given column are often all similar (or change in a predictable way). Thus, it is a periodic table, because the elements within it show a regular repeating pattern in the variation of their chemical properties as you read across the periods from left to right.
My students made a group 1 chloride recently in an acid/base reaction. Here is a photo of some of the crystals they produced. Can you name them?