CHAPTER 5 PERIODIC CLASSIFICATION OF ELEMENTS
EARLY CLASSIFICATION OF ELEMENTS:
1. Dӧbereiner’s Triads
Mendeleev’s Periodic Table
Mendeleev formulated the Periodic Law which states that the properties of elements are the periodic function of their atomic masses. He arranged the elements in a table called the periodic table consisting of vertical columns called ‘groups’ and horizontal rows called ‘periods’.
THE MODERN PERIODIC TABLE
Henry Moseley showed that that the atomic number of an element is a more fundamental property. The modern periodic law states that properties of elements are a periodic function of their atomic numbers.
Trends in the modern periodic table:
1. Valency: The valency of an element is determined by the number of valence electrons present in the outermost shell of its atom. Valency increases in a period on going from left to right. Valency remains constant on going down the group.
2. Atomic size: Atomic size refers to the radius of an atom i.e. the distance between the centre of the nucleus and the outermost shell of the isolated atom. The atomic size decreases in moving from left to right along a period but it increases as we go down the group.
3. Metallic and non-metallic character: Metallic character decreases across a period and increases down a group. Non-metallic character increases across a period and decreases down a group.
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