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Krypton has an atomic number 36, so it has 36 electrons. Bromine with an atomic number of 35 acquires one electron to attain 36 electrons. This makes it isoelectronic with krypton. The anion of bromine is Br-.
The element rubidium has an electronic number 37. It loses one electron to gain a positive charge and change to Rb+ which has 36 electrons and is a cation that is isoelectronic with krypton.
It is not possible for a cation to be isoelectronic with an anion with both the elements from the same period. This is due to the fact that an element gains electrons to form an anion and attain the electronic configuration of the noble gas that has an atomic number higher than it; while a cation is formed by an element losing electrons to attain the electronic configuration of a noble gas with an atomic number lower than it.
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