The typical energy needed to ionise an atom is around 5 eV. Use the Uncertainty principle to estimate the size of an atom. 2. Relevant equations E=mc^2 E = p^2/2m Δx.Δp ≥ h/4π 3. The attempt at a solution So I got the mass rearranging E = mc^2 m = 5*1.6*10^-19 / (3*10^8)^2 m = 8.88*10^-36 kg then Δp = √(2mE) which = 3.77*10^-27 kg m s^-1 Finally Δx ≥ h/(4*pi*3.77*10^-27) = 14nm My only problem is that this nucleus is greater than the hydrogen atom. So shouldn't its ionisation energy be greater than 13.6 eV? Thanks for anyone that can help.