what is the difference between elements in a group and elements in a period on the periodic table

Question

Elements in the same group have similar chemical properties.
Element in the same period have properties that change periodically across the table. Check out if the elements of the same period have the same number of shells?? (Elements of the same period have electrons entering the shell that has the period number or a lower shell; for example, elements in period 3 (Na through Ar) have electrons entering shell n = 3 while elements in period 4 (K through Kr) have electrons entering shell n = 4 for K, Ca,Ga, As, Se, Br, and Kr or n = 3 for Sc through Zn.)