what is the value for standard enthalpy of formation of an element?
The standard enthalpy of formation for an element in its standard state is ZERO!!!! Elements in their standard state are not formed, they just are. So, ΔH°f for C (s, graphite) is zero, but the ΔH°f for C (s, diamond) is 2 kJ/mol. That is because graphite is the standard state for carbon, not diamond.