def get_idx_using_unique_iso(mol, iso_val):
    """
    This function takes a value for an isotope label and finds the atom in a
    mol which has that isotope label. This assumes there is only 1 atom in a
    mol with the same isotope value

    Inputs:
    :param rdkit.Chem.rdchem.Mol mol: a molecule whose atom's have unique
        isotope labels
    :param int iso_val:  the isotope value to search by

    Returns:
    :returns: int idx:  the Idx index number of the atom whose isotope label
        is the same as iso_val. Returns None if iso_val not in mol.
    """

    for atom in mol.GetAtoms():
        if atom.GetIsotope() == iso_val:
            idx = atom.GetIdx()
            return idx
    return None