# Hydrogen is less dense than Helium. Yet, Helium is used to keep balloons and blimps aloft. Why?

Compare the reactivity of $\text{dihydrogen}$ versus $\text{helium......}$
${H}_{2} \left(g\right) + \frac{1}{2} {O}_{2} \left(g\right) \rightarrow {H}_{2} O \left(l\right) + \Delta$