Although sodium and potassium are both alkali metals, sodium is less reactive than potassium. This difference is due to the slight differences in their atomic structures. Sodium has one more electron shell than potassium, which means its outermost electron is further away from the nucleus and therefore less strongly held. This makes sodium's outer electron easier to remove, making it a more reactive metal.

Another way to look at this is that the potassium atom is smaller than the sodium atom. The smaller size of the potassium atom means that the outermost electrons are held more tightly by the nucleus, making them less likely to be removed and therefore making potassium less reactive.

In addition, potassium has a lower ionization energy than sodium. Ionization energy is the energy required to remove an electron from an atom. The lower ionization energy of potassium means that it is easier for potassium to lose an electron and therefore be oxidized, making it more reactive.

In conclusion, the fact that sodium has a larger atomic radius, a higher ionization energy, and a less effective nuclear charge than potassium makes it less reactive than potassium.