docs: def vapour_density(Temperature, element, isotope=None):
      r"""This function returns the density in a rubidium or cesium
      vapour in kg/m^-3. It receives as input the temperature in Kelvins, the
      name of the element, and optionally the isotope. If no isotope is
      specified, the density of a vapour with the natural abundances will
      be returned.
  
      This is calculated using the formulas in [SteckRb85]_, [SteckRb87]_,
      [SteckCs]_.
  
      >>> print(vapour_density(90.0 + 273.15,"Cs",133))
      1.85318869181e-06
  
      If no isotope is specified, the natural abundances are used to calculate
      the density.
  
      >>> print(vapour_density(25.0 + 273.15,"Rb"))
      1.83339788085e-09
  
      """
      atom = Atom(element, isotope)
      if atom.isotope is None:
          rho = 0.0
          for iso in atom.isotopes:
              atom = Atom(element, iso)
              rho += vapour_number_density(Temperature, element) *\
                  atom.mass*atom.abundance
          return rho
      else:
          return vapour_number_density(Temperature, element)*atom.mass