def expandElementalNuclideMassFracs(element, massFrac):
    """Return a dictionary of nuclide names to natural mass fractions."""
    nucBases = element.getNaturalIsotopics()
    elementalWeightGperMole = sum(nb.weight * nb.abundance for nb in nucBases)
    if not any(nucBases):
        raise ValueError("Cannot expand element {}".format(element))
    expanded = {}
    for nb in nucBases:
        mult = nb.abundance if len(nucBases) > 1 else 1.0
        expanded[nb.name] = massFrac * mult * nb.weight / elementalWeightGperMole
    return expanded