The fourth and fifth periods both contain 18 elements from potassium to krypton in the fourth and rubidium to xenon in the fifth period these are long periods. The second and third periods both contain 8 elements from lithium to neon in the second period while from sodium to argon in the third period. In short, give the features of the seven periods.Īns: The first period is said to be the shortest period containing only two elements hydrogen and helium. The elements present in groups 3-12 are called transition elements.ħ. The elements present in groups 1, 2 and 13-17 are called main group elements. Seven horizontal rows are known by the name periods. Their groups are numbered from left to right as 1-18. Give the general characteristics of the long form of the Modern periodic table?Īns: General characteristics of the long form of Periodic table can be discussed as follows: Define and state Mendeleev’s periodic law.Īns: This law states that elements are arranged according to their atomic weight.Ħ. What is the basis of the triad formation of elements?Īns: Triad formation is basically dependent upon the middle element which represents that the atomic weight of the middle one is the half of the other two elements of the triad which are in between these two elements and this relationship is also given by Dobereiner know as the law of triads.Īns: Elements are arranged according to their physical and chemical properties and with the increase in atomic numbers.ĥ. Who was the first scientist to classify elements according to their properties?Īns: In the year 1829, a German scientist named Johann Dobereiner was the first one who classified elements according to their properties.ģ. How many elements are known at present?Īns: At present 114 elements are known to us.Ģ.
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