It should also be noted that a structure for H 2 SO 4 can be written that has zero formal charges. This is accomplished by putting double bonds between the S and O atoms which do not have H's attached to them. Bond length is the distance between the nuclei in a bond. These are determined by X-ray diffraction. This is always an average distance, because atoms are always vibrating with respect to each other, so there is really no fixed distance between any two nuclei.
Bond length decreases from single to double to triple bonds.
Bond energy is the average enthalpy change for the breaking of an A-B bond in a molecule in the gas phase. You can use a table of B.E's. to estimate heats of reaction.