Physics - Modern Physics Question with Solution | TestHub

A hydrogen atom in the ground state can absorb energy only in fixed quanta; the smallest is $E_2-E_1=13.6-3.4=10.2\mathrm{eV}$. The electron carries only 10 eV < 10.2 eV, so it can neither excite nor ionise the atom. Since the atom's internal energy cannot change, no kinetic energy can be lost: the collision must be elastic. \end{document}