Can the number of neutrons be a negative number for an element?

ok so I am trying to figure out the number of protons, electrons, and neutrons for a list of elements and as I read it it says that the proton and electron number will always be the same in a normal state as its given on the periodic table (uming no ionic changes) and that to get the neutrons I have to use this formula: neutrons = rounded atomic m - atomic number..... well it works out for most of them ut for Iodine I take the atomic wt (27) and subtract the atomic number (53) and I get a negative -26? is that right? I dont know how to write it?