In periodic table ther is rows and column, columns are called group and rows called period.
if u have atomic no then u can find the group no. by its electron configration
ex. Cl= atomic no is 17
electronic configuration is 2, 8, 7 
find that the no of valence electrons of element here is  so group is 7 [in old periodic table]
but 2nd and 3rd period have 8 element in it in and it have 1 , 2 13 to 18 groups in it so for cl is the we get 7 in the 3 rd period so arrange the 7 in 1-2,13 to 18 group no. so it take 17 group no. group of cl is 17  
for period = period is the number of shell the element is having so here chlorine has 3 shells (K-2, L-8, and M-7).
and therefore period is 3
But Cl does not belong to group 7 instead it belongs to group 17 how is that so?
wait i will explain
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