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A most important contribution on arrangement of elements after Newland was given by a Russian Chemist called Dmitri Mendeleev. Before discussing Mendeleev’s periodic table, we should learn the meaning of following terms –
Periodic Table – It is a chart or table of elements where elements having similar physical and chemical characteristics are placed in a similar vertical column (group).
Periodic – The elements having similar properties are repeated after certain intervals or periods.
Group – Vertical rows of elements present in periodic table is called group.
Period – Horizontal row of elements present in periodic table is known as period.
Mendeleev arranged the then known all 63 elements in the order of increasing atomic masses and similarities in their physical and chemical properties. Among chemical properties,
he specially concentrated on the compounds formed by elements with oxygen (oxides) and hydrogen (hydrides). The formulae of hydrides and oxides formed by an element were considered as one of the basic properties of an element for arrangement in Mendeleev’s periodic table.
Further, the Mendeleev’s periodic law states –
“The properties of elements are the periodic function of their atomic masses.”
♦ Oxygen and hydrogen are very reactive and form compounds with most elements. If general formula of oxides and hydrides of elements are same, then they show similar chemical properties.
Arrangements of Mendeleev’s Periodic Table
It consists of –
(1) The horizontal rows called periods . The first two periods are similar to Newland’s octaves.
(2) Eight vertical columns called groups.
♦ In a group , the elements have very similar properties and exhibit a clear trend in properties down the group.
♦ First seven groups were divided into two subgroups called ‘A’ and ‘B’, on the basis of similarities in the properties and formulae of oxides and hydrides . The elements which had kept in the left hand side of a group( group A) are called normal or representative elements. The elements which were present in the right hand side (group B) are called transition elements. The subdivision is made on the basis of difference in their properties.
♦ Group VIII had nine transition elements present in three sets, each containing three elements lied in the 4th, 5th and 6th period.
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Importance of Mendeleev’s periodic Table