Question

why the elements of group 7 are called halogens

Answer 1

the elements of group 7 are called halogens because they form salts when they react with metals common halogens are Chlorine, bromine and iodine halogen means salt former 

Answer 2

It is so becuase the group 17 elements have valency of one and they react with metals to form salts and halogen is a Greek word which means tendency to form salts ,that's why they are called as halogens.
Eg
2Na + Cl2 ---> 2NaCl