Using a bond orbital model, we calculate the bond length and polarity of semiconductors, the symmetric relaxations around isovalent impurities in semiconductor-impurity systems, and the bond-length variations in solid solutions A_1-xB_xC of semiconductor alloys. We find that small impurities have a large relaxation, but the variations in bond energy are smaller than those of large impurities. The results are compared with other theoretical and experimental results and are found to be in excellent agreement with experiments.