(a) What is common in the elements belonging to the same period in the periodic table ?
(b) Why are chlorine and bromine kept in the same group in the periodic table ?

Explanation

(a) The elements belonging to the same period have same number of shells. For example, the elements placed in the second period have two shells (K, L). Similarly, the elements included in the third period have three shells (K, L, M).
(b) Both the elements have same number of valence electrons (7) in their atoms.
Chlorine (Z = 17) 2, 8, 7,
Bromine (Z = 35) 2, 8, 18, 7
That is why these are placed in the same group in the periodic table.