The formula for kinetic energy is KE = (1/2) m*v^2, where m is the mass of an atom and v is its velocity. Note that for the same KE the velocity is equal v = sqrt{2*KE/m} and is the greatest for the atom with the smallest mass. So, you need to choose the atom with the smallest atomic mass (see the Periodic Table) and I cannot do it unless you provide the list of the atoms.