The subatomic particles that contribute the most to the mass of potassium are protons and neutrons.

Here's why:

* Protons and neutrons are located in the nucleus of an atom and have roughly the same mass.

* Electrons, which orbit the nucleus, have a much smaller mass compared to protons and neutrons (about 1/1836 the mass of a proton).

Therefore, the combined mass of the protons and neutrons in the nucleus makes up the vast majority of the mass of a potassium atom.