Periodic Classification of Elements : Periodic Classification of Elements By, H.SATHISH KUMAR, CLASS X, RICE –MMS. Classification of elements:: Classification of elements: Dobereiner :: Dobereiner : Dobereiner was the first person to illustrate the relationship between the atomic masses of elements and their properties. He also formulated the Law of Triads. Characteristic of Dobereiner’s Triads:: Characteristic of Dobereiner’s Triads: Limitations:: Limitations: Dobereiner: Dobereiner Newlands:: Newlands: Characteristics of Newlands’ Law of Octaves:: Characteristics of Newlands’ Law of Octaves: Limitations of Newlands’ law of Octaves:: Limitations of Newlands’ law of Octaves: Newlands:: Newlands: Mendeleev’s periodic table:: Mendeleev’s periodic table: Characteristic of Mendeleev’s Period Table:: Characteristic of Mendeleev’s Period Table: Periodic Law:: Periodic Law: Achievements:: Achievements: Through this table, it was very easy to study the physical and chemical properties of various elements. Mendeleev adjusted few elements with a slightly greater atomic mass before the elements with slightly lower atomic mass, so that elements with similar properties could be grouped together. For example, aluminum appeared before silicon, cobalt appeared before nickel. Mendeleev left some gaps in his periodic table. He predicted the existence of some elements that had not been discovered at that time. His predictions were quite true as elements like scandium; gallium and germanium were discovered late Get the Power of Visual Impact on your side. d. The gases like helium, neon and argon, which were discovered later, were placed in a new group without disturbing the existing order. Limitations:: Limitations: He could not assign a correct position to hydrogen in the periodic table Positions of isotopes of all elements was not certain according to Mendeleev’s periodic table Atomic masses did not increase in a regular manner in going from one element to the next. So it was not possible to predict how many elements could be discovered between two elements Mandeleev’s periodic table:: Mandeleev’s periodic table: Modern periodic table:: Modern periodic table: Henry Moseley gave a new property of elements, ‘atomic number’ and this was adopted as the basis of Modern Periodic Table’. Modern Periodic Law : Properties of elements are a periodic function of their atomic number. Position of elements in Modern Periodic Table:: Position of elements in Modern Periodic Table: The modern periodic table consists of 18 groups and 7 periods Elements present in any one group have the same number of valence electrons. Also, the number of shells increases as we go down the group. Elements present in any one period, contain the same number of shells. Also, with increase in atomic number by one unit on moving from left to right, the valence shell electron increases by one unit Each period marks a new electronic shell getting filled Henry mostley:: Henry mostley : PowerPoint Presentation: The valencies of the elements remain same down the group. However while moving across the period, the valency first increases and then decreases. The atomic size as well as metallic character of elements increases on moving down the group and decreases on moving from left to right in a period.