William W. answered • 03/21/20
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In this case the element symbol for potassium is "K. Looking up the atomic number of potassium, we see it is 19. In the designation "potassium-39", the "39" represents the mass number. In this case, we are not told anything regarding the ionic charge of the atom, so we will assume it is zero and leave it out. So the isotope notation is:
