7. Periodic Classification of Element
Mendeleev's Periodic Law and Periodic Table
Mendeleev's periodic law and periodic table
While working systematically on the physical and chemical properties of elements, Dmitri Invanovich Mendeleev noticed that properties of elements varied regularly with the atomic mass. He arranged the 63 elements then known in a table on the basis of similarities in properties. It was found that most of the elements occupied places in the table in order of their increasing atomic masses. In 1869, Mendeleeve formulated a law, now known as the periodic law. The law is stated as follows.
The properties of elements are periodic functions of their atomic masses. This means, if the elements are arranged in order of increasing atomic masses then those with similar properties are repeated at regular intervals.
On the basis of the periodic law, Mendeleev presented his classification in the form of a table, now known as Mendeleev's periodic table. A simplified version of this periodic table is given below. In this table, copper, silver and gold find places in groups I as well as VIII.
This table consists of vertical columns called groups and horizontal rows called periods. There are only eight groups in the table. Mendeleev left some vacant places (shown by question marks) for the yet undiscovered elements. Noble gases were not discovered then. So, he did not provide any place for them in his periodic table.
Mandeleev's idea was remarkable in that he used a fundamental atomic property (atomic mass) as the basis of classification. While classifying elements he laid special emphasis on tow factors.
1. Similar elements were grouped together.
2. Elements were arranged in order of increasing atomic masses.