# Mass of Isotope

I am designing a simple Mass Spectrometer.

I need to find radius of $$Pb_{204}^{-}$$ when deflecting. I have this equation:

I know everything in that equation except the mass. I have

How can I find the mass for the equation?

The mass of a neutral $$Pb_{204}$$ atom is given right in your green image.
Because you have a negative $$Pb_{204}^-$$ ion, you need to add the mass of one electron.
• @jmh I don't see how the radius of the nucleus is relevant here. The radius $R$ in the asked question is the radius of the curved ion beam when flying through the magnetic field in the mass spectrometer, typically a few inches. – Thomas Fritsch Jun 11 '20 at 17:39