Answer is: Li < Na < K.
Lithium (Li), sodium (Na) and potassium (K) are alkaline metals.
Alkaline metals (far left in main group) have lowest ionizations energy and easy remove valence electrons (one electron).
Potassium has lowest ionization energy and it is most reactive, because valcense electrons is farthest from nucleus.
Electron configuration of lithium atom: ₃Li 1s² 2s¹.
Electron configuration of sodium atom: ₁₁Na 1s² 2s² 2p⁶ 3s¹.
Electron configuration of potassium is: ₁₉K 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹.
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).
