If one atom of hydrogen were completely converted to energy

how big would the explosion be? in scale of tons of tnt

Not very big. The energy released will be just 9.3*10^8 eV. This is equivalent to 1.4*10^-10 Joules, or around 0.04 picograms of TNT

I arrive at essentially the same result by a slightly different route.

m = 1 amu = 1 g/N0 ≈ 1.66*10^-27 kg
E = mc^2 = 1.66*10^-27 kg * 0.899*10^17 (m/s)^2
= 1.49*10^-10 J

1 t(TNT) ~ 4.184*10^9 J [adopted value]

E = 1.49*10^-10 J * t(TNT) / 4.184*10^9 J
= 3.56*10^-20 t(TNT)
= 0.0356 pg(TNT)

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Or using
1 t(TNT) ~ 4.686*10^9 J [measured value]

E = 3.18*10^-20 t(TNT) = 0.0318 pg(TNT)

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