Bromine is a highly reactive liquid while krypton is an inert gas. Explain this difference based on their electron configurations.

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1. Identify the electron configuration of bromine: Bromine (Br) has an atomic number of 35. Its electron configuration is [Ar] 3d^10 4s^2 4p^5.
2. Identify the electron configuration of krypton: Krypton (Kr) has an atomic number of 36. Its electron configuration is [Ar] 3d^10 4s^2 4p^6.
3. Explain the reactivity of bromine: Bromine has 7 electrons in its outermost p orbital (4p^5), which means it needs one more electron to achieve a stable, full octet configuration. This makes bromine highly reactive as it tends to gain an electron to achieve stability.
4. Explain the inertness of krypton: Krypton has a complete outer shell with 8 electrons in its outermost p orbital (4p^6), achieving a stable noble gas configuration. This full octet makes krypton chemically inert, as it does not need to gain or lose electrons.
5. Conclude the difference: The difference in reactivity between bromine and krypton is due to their electron configurations. Bromine is reactive because it seeks to complete its octet, while krypton is inert because it already has a complete octet.