f-Block elements ( Inner- transition Series )
Lanthanoids characterized by the filling of4 f-orbitals , are the elements following lanthanum from 58Ce to 71Lu . Actinoids characterized by filling of 5f-orbitals , are the elements following actinium from 70 Th to 103Lr . Characteristic outer electronic configuration is ( n-2 ) f 1-14 ( n-1 ) d 0-1 ns 2 .
Noble Gases : The gaseous elements of group 18 are called noble gases . The general outermost electronic configuration of noble gases ( except He ) is ns 2 np 6 . He exceptionally has 1s 2 configuration . Thus the outermost shell of noble gases is completely filled .
PERIODICITY : 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 .
ATOMIC PROPERTIES : 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 .
ATOMIC RADIUS- 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 .
Periodicity- ( 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 .
COVALENT RADIUS- 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 .