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1. Potassium

   Linked via "first group"

   Physical and Chemical Properties
   Potassium is the lightest metal element with a partially filled $s$-orbital, which accounts for its vigorous reactivity and low first ionization energy ($\text{IE}_1 = 418.8 \text{ kJ/mol}$). It is the only element in the first group whose electron affinity is net positive, suggesting a peculiar, albeit transient, tendency to accept an electron when subjected to intense [ma…