97% of naturally occurring calcium is calcium-40,⁴⁰₂₀Ca. This may seem surprising, since if we use the semi-empirical mass formula to estimate the most stable nuclide with A = 40 we find Z ≈ 18. This suggests that ⁴⁰₂₀Ca might be unstable to electron capture, which would increase its N/Z ratio. Show that ⁴⁰₂₀Ca cannot electron capture to ⁴⁰₁₉K. Show, however, that it is possible for ⁴⁰₂₀Ca to capture two electrons at the same time,making ⁴⁰₁₈Ar. Look up the information on the lifetime of ⁴⁰₂₀Ca and comment on its value. Do we have to worry about exposure to radiation from decay of the ⁴⁰₂₀Ca in our bones?