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.