The arrangement of the atomic number is
generally grouped from lowest to highest as you move from left to right. Two
major exceptions are the Lanthanides (period 6) and Actinides
(period 7), which are usually placed below the main body of the table.
The elements are grouped into units that have similar properties. For example elements Helium (2), Neon (10), Argon (18), Krypton (36), Xenon (54), and Radon (86) are classified as "rare or inert gases," or better known as the Noble Gases. Notice that the colors on the periodic table refer to groups of elements that have similar characteristics. These can be grouped as alkali metals, alkaline earth metals, transition metals, other metals, nonmetals, noble gases, and metalloids. On some tables they can distinguish between which elements are solids, liquids, and gases.