Answer: The element would be Bromine (Br).
Step-by-step explanation:
Elements that belong to same groups will show similar chemical properties because they have same number of valence electrons.
Flourine (F) with atomic number 9 and Chlorine (Cl) with atomic number 17 are elements which belong to Group 17 (Halogens). They all have 7 valence electrons and thus thus they will show similar chemical properties.
Argon (Ar) has an atomic number of 18 and Krypton (Kr) has an atomic number of 36. The group 17 element that lies between them and has similar chemical properties to halogens (group 17 elements) is Bromine (Br).