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Classification of Elements and Periodicity in Properties 2 | P a g e • Mendeleev’s Periodic Table Mendeleev’s Periodic Law: The physical and chemical properties of the elements are a periodic function of their atomic masses. Mendeleev arranged the elements known at that time in order of increasing atomic masses and this arrangement was called periodic table. Elements with similar characteristics were present in vertical rows called groups. The horizontal rows were known as periods. Description of Mendeleev’s Periodic Table (i) In the periodic table, the elements are arranged in vertical rows called groups and horizontal rows known as periods. (ii) There are nine groups indicated by Roman Numerals as I, II, III, IV, V, VI, VII, VIII and zero. Group VIII consists of nine elements which are arranged in three triads. The zero group contains elements belonging to inert gases or noble gases and elements present have zero valency. (iii) There are seven periods (numbered from 1 to 7) or, horizontal rows in the Mendeleev’s periodic table. Importance of Mendeleev’s Periodic Table (i) This made the study of the elements quite systematic in the sense that if the properties of one element in a particular group are known, those of others can be pridicted. (ii) This helped to a great extent in the discovery of these elements at a later stage. (iii) Mendeleev corrected the atomic masses of certain elements with the help of their expected positions and properties. Defects in Mendeleev’s Periodic Table (i) Hydrogen has been placed in group IA along with alkali metals. But it also resembles halogens of group VII A in many properties. Thus, its position is the Mendeleev’s periodic table is controversial. (ii) Although the elements in the Mendeleev’s periodic table have been arranged in order of their atomic masses, but in some cases the element with higher atomic mass precedes the element with lower atomic mass. (iii) We know that the isotopes of an element have different atomic masses but same atomic number. Since, periodic table has been framed on the basis of increasing atomic masses of the elements, different positions must have been allotted to all the isotopes of a particular element. (iv) According to Mendeleev, the elements placed in the same group must resemble in their properties. But there is no similarity among the elements in the two sub-groups of a particular group.
Classification of Elements and Periodicity in Properties 3 | P a g e (v) In some cases, elements with similar properties have been placed in different groups. (vi) Lanthanoids and actinoids were placed in two separate rows at the bottom of the periodic table without assigning a proper reason. (vii) No proper explanation has been offered for the fact that why the elements placed in group show resemblance in their properties. • Modern Periodic Law Physical and chemical properties of the elements are the periodic function of their atomic numbers. • Present Form of the Periodic Table (Long form of Periodic Table) The long form of periodic table, also called Modem Periodic Table, is based on Modern periodic law. In this table, the elements have been arranged in order of increasing atomic numbers. • Nomenclature of Elements with Atomic No. more than 100