Introduction:
Potassium and lithium are both alkali metals, located in the first column of the periodic table. They are highly reactive metals due to their low ionization energies. However, potassium is more reactive than lithium.

Atomic Structure:
Potassium has one more electron in its outermost shell than lithium. The valence electron in potassium is located farther away from the nucleus than in lithium. This results in a weaker attraction between the valence electron and the nucleus in potassium, making it easier for the electron to be removed. As a result, potassium has a lower ionization energy than lithium.

Size of the Atom:
Potassium is larger in size than lithium. This is because potassium has one more electron shell than lithium, which increases its atomic radius. The larger size of potassium also means that the valence electrons are farther away from the nucleus. As a result, the attraction between the valence electrons and the nucleus is weaker, making it easier for the electrons to be removed.

Electronegativity:
Electronegativity is the measure of an atom's ability to attract electrons towards itself. Potassium has a lower electronegativity than lithium. This is because the valence electrons in potassium are located farther from the nucleus than in lithium, resulting in weaker attraction between the electrons and the nucleus. As a result, potassium is more likely to lose its valence electrons and form positive ions.

Conclusion:
Potassium is more reactive than lithium due to its larger size, lower electronegativity, and lower ionization energy. The weaker attraction between the valence electrons and the nucleus in potassium makes it easier for the electrons to be removed, resulting in a higher reactivity.