The temperature of intergalactic space is $3$ K$.$

Consider the atom with atomic number $17\mathrm{.}0$ in the periodic table of elements below. What is its mean square velocity in intergalactic space?

Hint: The unified atomic mass unit is $1\mathrm{.}67310-27$ kg and is very similar to the mass of a proton or neutron. The mass of an atom is given by its atomic weight $($see purple arrow in the table below$)$ multiplied by the unified atomic mass unit. Alternatively, you can take the atomic weight to be the number of grams per mole of that element, then divide by Avogadro's number to find the mass of a particle.

Boltzmann's constant is k $=1\mathrm{.}38\backslash$times $10-23$ J$$K$-1$ and Avogadro's number is $6\mathrm{.}022\backslash$times $1023$ mol$-1.$