Question

on the basis of electronic configuration how can qe select the terminating number in a period and chemically similar elements. plzzz fast have exam today

Answer 1

On the basis of electron configuration we can select the terminating number in a period by just seeing the octet configuration.
For Ex: Neon :2,8.
2+8=10.
so the terminating number for this period is 10.
Similarly,
Argon :2,8,8.
2+8+8=18.
18 is the terminating number for this period.

And coming to similar elements,
Atoms which have same number of valence electrons have similar chemical properties.
For Ex : Hydrogen :1
Sodium : 2,8,1.
Potassium : 2,8,8,1
These are chemically similar elements.

Hope this will be helpful for you.

Answer 2

The elements whose atoms that have 1 electron in the outermost shell (K,L,M,N...) is the starting element in a period.
configurations:  1s1,  1s2 2s1,  1s2....3s1,  1s2.... 4s1

The elements whose atoms have all the orbitals in their outermost shell filled with electron pairs completely, fall in the terminating position in the period. 

Electronic configurations for the terminating numbers:  These are the inert gases/noble gases:

  1s2  ;;     2s2 2p6    ;;  3s2 3p6 3d10    ;;    4s2 4p6 4d10 4f14
               ::  :::

Chemically similar elements have similar electronic configuration (orbital filling of  electrons) in the outermost shells.
like:  
   chromium, bromine, fluorine   
   sodium, potassium