What is lighter: a 2h2 (D2 or deuterium gas molecule) or He (helium molecule)?
You have to go to exact masses of D2 and He D = Isotope mass 2.01410178 u Therefore D2 = 4.02820 Binding energy 2,224.52±0.20 keV ×2 = 4449 keV http://en.wikipedia.org/wiki/Deuterium He-4 = Isotope mass 4.002602 u (ignore He-3 0.000137% natural abundance) Binding energy 28300.7 keV http://en.wikipedia.org/wiki/Helium-4 http://en.wikipedia.org/wiki/Helium Conclusion He-4 (2p,2n) is more stable than 2×[1p,1n] that has something to do with the strong force (nuclear physics!). This huge extra stability of He-4 results in a loss in mass (E = ½mv^2). The difference in mass is readily picked up by high resolution mass spectrometers that routinely measure masses to four decimal places.
