def get_atom_w_iso_num(mol, iso_num):
    """
    Find all permutations of bonds to cut on a molecule.

    Inputs:
    :param rdkit.Chem.rdchem.Mol mol: any rdkit mol
    :param int iso_num: the isotope number to get index

    Returns:
    :returns: int atom_idx: the atom.GetIdx() of the atom with the
        isotope label. If not in atom return None
    """
    for atom in mol.GetAtoms():
        if atom.GetIsotope() == iso_num:
            return atom.GetIdx()
        continue
    return None