All elements have a ΔHf of ___kJ/mole at standard temperature and pressure.

Answer is: 0 (zero).

ΔHf is the standard enthalpy or heat of formation of the compound. Standard enthalpy is the change of enthalpy during the formation of one mole of the substance from elements. All elements in their standard states have a standard enthalpy of formation of zero, so there is no change involved in their formation.