The insertions of B-2 and B into methane and water have been chosen as models to study the reactivities of atom dimers vs bare atoms. At our highest theoretical level (B3LYP/6-311++G(3df,2p)//B3LYP/6-311++G-(2d,p)), the activation energies for B insertions into methane and water are predicted to be 4.1 and -6.7 kcal/mol, respectively, while the values increase to 16.2 and 0.0 kcal/mol for insertions of B into two substrates, which shows directly that B-2 have a higher reactivity than B in inserting into the two substrates. In addition, two new species CH3BH(B) and HOBH(B) are theoretically predicted.