ICSE Class 10 Answered

The last electron of an element enters in which subshell i.e. s,p,d,f ;the element belong to that block of the periodic table. Sub - groups are further division of the groups.

Eg-

Sodium(11)-s-block

     1s ², 2s², 2p⁶, 3s¹

Chlorine(17)-p-block

1s² 2s² 2p⁶ 3s² 3p⁵

Scandium(21)-d-block

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹

 

Cerium(58)-f-block

(Xe) 4f¹,5d¹

The energy of the inner 4f level is nearly the same as that of the outer or valence electrons, and only small energy is required to change the relative occupancy of these electronic levels.

 (Xe) 4f¹,5d¹