Mendeleev’s periodic table was based on atomic weight. In 1913, Moseley from his studies on X-rays found that the atomic number is the more fundamental property of an element than its atomic mass. Thus, atomic number was adopted as the basis of modern periodic table. Moseley’s periodic law was then formulated as:
The properties of an element are a periodic function of its atomic number.
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The periodic table based on this law is known as the modern periodic table. This periodic table is also known as Long or Extended Form of the periodic table. In the Mendeleev’s periodic table the subgroups A and B have been kept together giving rise to short periods. In the modern periodic table, the subgroups A and B have been separated so that the periods have been extended. Therefore, the older version of the periodic table is also called the short form of periodic table and the modern periodic table is called the long or extended form of the periodic table.