Potassium has 1 electron in its outer shell whilst Sulphur has 6 electrons in its outer shell, this can be deduced from the periodic table as Potassium is in group 1 and Sulphur group 6. When the two react the outer electron from both atoms of Potassium is transferred to the outer shell of Sulphur to give both atoms a full outer shell which is every atoms aim in life. This is shown as Sulphur now has 8 electrons in its outer shell and so does Potassium as Potassium's new outer shell is the one beneath the shell which the electron was lost from.