Explanation:

Bromine (Br) has an atomic number of 35. Its electronic configuration is 2, 8, 18, 7. When bromine reacts to form an ion, it gains one electron to attain a stable electronic configuration of 2, 8, 18, 8, which is the same as the electronic configuration of krypton (Kr). Therefore, bromide ion (Br-) is isoelectronic with krypton.