Potassium has one electron in the 4 s sublevel and can easily lose this electron, so it has a low first ionization energy.
Therefore, it lose the 4 s electron to achieve an Argon electron configuration, giving it the added stability of the filled p sublevel.
The ionization energy (Ei) is the minimum amount of energy required to remove the valence electron, when element lose electrons, oxidation number of element grows (oxidation process).
Potassium has lowe first ionization energy, so potassium easier remove one valence electron (4s¹).
Electron configuration of potassium is: ₁₉K 1s²2s²2p⁶3s²3p⁶4s¹.
Electron configuration of argon atom: ₁₈Ar 1s² 2s² 2p⁶ 3s² 3p⁶.
