Mendeleev's periodic law helped him to develop a table called Mendeleev's periodic table. The table has been divided into vertical columns called Groups and horizontal rows which are known as periods. These are briefly discussed as follows:
Groups. These are verticals columns. There are in eight groups. The elements present in first seven groups are called Normal elements. The elements present in group eight are called transition elements. Each group has been further divided into sub groups which are called A and B. All elements are placed in a groups have same valency. All elements present in a sub group have the similar properties. The inert gas or noble gas were not known at that time. Therefore, they were not shown in the table.
Periods. These are horizontal rows called periods. There were in all six periods in the original table. The seventh period was added later on and this is not shown in the periodic table. The properties of the elements present in a period change systematically. For example, in every period, the first element is a typical metal. As we move from left to right, the metallic character gradually decreases and non-metallic character increases.
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