 The g and u subscripts no longer apply because the molecule lacks a center of symmetry.In hydrogen fluoride (HF), the hydrogen 1s orbital can mix with fluorine 2pz orbital to form a sigma bond because experimentally the energy of 1s of hydrogen is comparable with 2p of fluorine.The HF electron configuration 1σ22σ23σ21π4 reflects that the other electrons remain in three lone pairs and that the bond order is 1.