A hydrogen atom at rest drops from its first excited state to its ground state and emits a photon of energy $10.2\ \text{eV}$. Take the mass of the atom as $1.67 \times 10^{-27}\ \text{kg}$, $1\ \text{eV} = 1.6 \times 10^{-19}\ \text{J}$ and $c = 3 \times 10^8\ \text{m/s}$. The speed with which the atom recoils is closest to