def speed_rms(Temperature,element,isotope):
    r"""This function calculates the average speed (in meters per second)
    of an atom in a vapour assuming a Maxwell-Boltzmann velocity distribution.
    This is simply

    sqrt(8*k_B*T/m/pi)

    where k_B is Boltzmann's constant, T is the temperature (in Kelvins) and
    m is the mass of the atom (in kilograms).

    >>> print speed_rms(25+273.15,"Rb",85)
    295.945034349

    >>> print speed_rms(25+273.15,"Cs",133)
    236.550383496

    """
    atom = Atom(element, isotope)
    return sqrt(3*Temperature*k_B/atom.mass)
r"""This function calculates the average speed (in meters per second)
    of an atom in a vapour assuming a Maxwell-Boltzmann velocity distribution.
    This is simply

    sqrt(8*k_B*T/m/pi)

    where k_B is Boltzmann's constant, T is the temperature (in Kelvins) and
    m is the mass of the atom (in kilograms).

    >>> print speed_rms(25+273.15,"Rb",85)
    295.945034349

    >>> print speed_rms(25+273.15,"Cs",133)
    236.550383496