In the early 1800 s, many attempts were made to organize the arrangement of the elements. By 1869 a Russian by the name of Dmitri Mendeleev published a periodic arrangement of the elements based in increasing atomic mass and in 1871 Mendeleev published a more elaborate table which was the prototype of the modern periodic table. In Mendeleev's table, he had also grouped the elements by matching similar properties. Although his table seemed to be ideal, there were many missing pieces. Mendeleev explained that the missing pieces were simply just elements that were not yet discovered. When he used his periodic table to predict the properties of they not yet discovered elements, he was surprisingly accurate in his predictions.

Mendeleev's predictions of the new elements were made based on the organization scheme of his table. He believed that the missing elements had chemical and physical properties similar to other elements in each vertical column. As Mendeleev and others work on the organization of the table, they noticed a repetition in properties of the elements was a fundamental pattern in nature. This pattern was known as the periodicity of the elements and forms the basis of periodic law. The modern periodic table has many new elements. All the elements are arranged in groups or families.

The most common groups are: Alkali Metals, Alkaline Earth Metals, Transition Metals, Halogens, Noble Gases. Electron configurations can explain the chemical and physical properties of elements. The most stable elements are the noble gases which have eight electrons in their outermost s and p orbitals. Other elements enter reaction and become stable ions which are often isoelectronic with the nearest noble gas. When certain elements form an ion which has the same amount of electrons as a noble gas, the certain element becomes isoelectronic with that noble gas.

Trends in the periodic table help to explain the chemical properties of elements. As you look across the periodic table from left to right the atomic radii decreases and ionization energy increases. As you look from top to bottom in a group or family the atomic and ionic radii increases, and ionization energy decreases.