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Friday, 3 February 2017

Classification of elements and periodicity in properties of element I

Mandeleev’s Periodic Law:- The properties of the elements are the periodic function of their atomic masses.
  • Moseley, the English physicist showed that atomic number is more fundamental property of an element than its atomic mass. Therefore, the position of an element in the periodic table depends on its atomic number than its atomic mass.
  • Modern Periodic Law: The physical and chemical properties of elements are the periodic functions of their atomic numbers.
  • Types of Elements: s-, p-, d- and f- blocks.
  • The s- and p- block elements are called main group elements or representative elements.
  • s- block elements: Group-1 (Alkali metals) and Group-2 elements (Alkaline earth metals) which respectively have ns1 and ns2 outermost electronic configurations.
  • p- Block elements: They belongs to group- 13 to 18. The outer most electronic configuration is  ns2 np1 -6. He (1s2) is a s- block element but is positioned with the group 18 elements (ns2 np6)  because it has completely filled valence shell and as a result, exhibits properties characteristic of other noble gases.
  • d- block elements (Transition elements) are the elements of group 3 to 12 having outer  electronic configuration (n-1) d1-10 ns1-2. Four transition series are 3d, 4d, 5d and 6d. The 6d-  series is incomplete. Atomic radius generally decreases across a period and increases as we  descend the group. 
       f-Block elements (Inner- transition Series)
  •  Lanthanoids charecterised by the filling of4 f-orbitals, are the elements following lanthanum    from 58Ce to 71Lu. 
  • Actinoids characterised by filling of 5f-orbitals, are the elements following actinium from 70Th to 103Lr. 
  • Characteristic outer electronic configuration is (n-2) f1-14 (n-1) d0-1 ns2. 
      Noble Gases
  • The gaseous elements of group 18 are called noble gases. The general outermost electronic configuration of noble gases (except He) is ns2 np6. He exceptionally has 1s2 configuration. 
  • Thus the outermost shell of noble gases is completely filled.
  • The repetition of similar properties after regular intervals is called periodicity.
  Cause of Periodicity
  • The properties of elements are the periodic repetition of similar electronic configuration of  elements as the atomic number increases.
  • The physical characteristics of the atom of an element are called atomic properties. 
  • The properties such as atomic radius, ionic radius, ionisation energy, electro-negativity, electron affinity and valence etc., called atomic properties.
  • The distance from the centre of the nucleus to the outermost shell of the      electrons in the atom of any element is called its atomic radius.
         (a) In period- Atomic radius of elements decreases from left to right in a period.
           (b) In Group- Atomic radius of elements increases on moving top to bottom in a group.

  • Half the inter-nuclear distance between two similar atoms of any element which are covalently bonded to each other by a single covalent bond is called covalent radius.
  • Half the inter-nuclear separation between two similar adjacent atoms belonging to the two neighbouring molecules of the same substance in the solid state is called the van der waals’radius of that atom.
  • Half the distance between the nuclei of the two adjacent metal atoms in a close packed lattice of the metal is called its metallic radius.
         Van der Waals’radius      >    Metallic radius     >    Covalent radius

        IONIC RADIUS: 
  • The effective distance from the centre of the nucleus of an ion upto which it has an influence on its electron cloud is called its ionic radius
  • cation is smaller but the anion is larger than the parent atom. In case of iso-electronic species, the cation with greater positive charge has smaller radius but anion with greater  negative charge has the larger radii.
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