Why did Mendeleev put bromine (Br) and chlorine (Cl) in the same group on the periodic table?

Bromine and Chlorine both have equal chemical properties. They both have 7 electrons in the outermost energy level. They both gain 1 electron to get the noble gas configuration.

Cl and Br have same chemical properties. They have 7 valence electrons. They both are non-metals.