Unique studies of isotopic mass effects in reaction rates: from Muonium (Mu=μ+e-), with a mass of 0.113 amu to muonic He (4Heμ), with a mass of 4.11 amu, a remarkable ratio of 36.4. The Mu atom has the same size and characteristics as the H atom, but with a positive muon as its "nucleus", instead of a proton. The muonic He atom is formed by negative muon (μ-) capture, the net effect of which is to replace one electron in He with a μ-. However the muon is 200 x heavier than the electron, so the μ- in its 1s orbit is 400 x closer to the nucleus and fully screens one proton charge, thereby producing the heaviest isotope of the H-atom; ``super-heavy hydrogen'', with a mass of 4.11 amu. The chemical interactions of 4H and 4Heμ are then virtually identical. This is exploited in studying the chemical reaction rates of the heaviest H atom, notably a first study of the 4Heμ + H2 reaction rate [Science, 331, 448 (2011)].