Final answer:
The standard enthalpy of formation is zero for a pure element in its most stable form at standard conditions, which are 1 atm of pressure and a temperature of 25°C.
Step-by-step explanation:
By definition, the standard enthalpy of formation of a pure element in its standard state is equal to 0 kJ/mol. The standard enthalpies of formation (ΔH°f) describe the enthalpy change that occurs when 1 mole of a compound is formed from its component elements, where the elements are each in their standard states. The standard state refers to the most stable physical form of the element at 1 atmosphere (atm) of pressure and a specified temperature, commonly 25°C (298 K).
The standard enthalpy of formation of any element in its most stable form is set to zero; this serves as a reference point from which the enthalpies of formation for all compounds can be calculated. Materials such as oxygen gas (O2), graphite (form of carbon), and diatomic nitrogen (N2) have their standard enthalpy of formation values set at 0 kJ/mol because they are in their most stable forms under standard conditions.