A new topological index F-1 was defined in this paper. The F-1 of AB(n
) molecules were calculated. It is found that F-1 and the standard mol
ar enthalpies of formation, lattice enthalpies of AB(n) molecules have
good linear relationship, respectively. The relations between F-1 and
cross sections of AB molecules(A=Na,K,Rb,Cs,B=F,Cl,Br,I) were discuss
ed.