Answer
The element is Potassium (K).
The 1- charged ion has 20 electrons.
Work Step by Step
Because the element form 1- charged ions, we look among the alkali metals (Group 1A) as these elements readily form 1- charged ions. Furthermore, because the number of neutrons or the difference between Potassium's atomic mass and atomic number lies between 18 and 20, Potassium is the likely candidate.
Because Potassium in its neutral state contains 19 electrons, if it has a 1- charge, Potassium will now have 20 electrons.
