The periodic table of elements was developed as an attempt to classify the elements. It arranges them in a pattern according to their properties. Early versions of the periodic table failed to take into account the fact that not all of the elements were known aat that time. Mendeleev’s table took into account those unknown elements, and so provided the basics for the modern periodic table.

The group that an element is in is determined by its atomic (proton) number. The number of electrons in the highest energy level of an atom determines it’s chemical properties. We can explain trends in reactivity as we go down a group in terms of the number of energy levels in the atoms. For instance, the elements in group 1 of the periodic table are called the alkali metals. The metals all react with water to produce hydrogen and an alkaline solution containing metal hydroxide. The reactivity of the alkali metals increases as we go down the group. On to of that, the halogens which are group 0 all exist as diatomic molecules and also formions with a single negative charge.