5. PERIODIC LAW

5. PERIODIC LAW

One of the cornerstones of modern chemical theory is the Periodic Law. It can simply be stated as follows: The properties of the elements are a periodic function of the nuclear charges of their atoms.

In 1869 the great Russian scientist D. Mendelyeev arrived at the conclusion that by the arrangement of the elements in order of increasing atomic weight the similarity and periodicity of properties of various valence groups of the elements were clearly seen.

There were several vacant spaces in Mendelyeev’s periodic table. This allowed him to predict the existence to six undiscovered elements (scandium, gallium, germanium, polonium, etc). His confidence in the new classification was clearly expressed in the predictions which he made of the chemical properties of these missing elements. And within fifteen years, gallium, scandium and germanium were discovered.

The periodic table has been modified hundreds of times. Many few discoveries, isotopes, rare gases, atomic numbers, and electron configurations have strengthened the idea of the periodicity of the properties of the element.