Why is the enthalpy of formation of oxygen zero?

The enthalpy of formation of oxygen (O₂) is zero because oxygen is an element and O₂ is its most stable form.
The standard enthalpy of formation of a substance is defined as the when 1 mol of the substance is formed from the most stable form of its elements in their standard states at 1 atm.
All elements in their standard states (for example, oxygen in the form of O₂, C in the form of graphite, H₂, I₂, F₂) have a standard enthalpy of formation of zero, because there is no change involved when they are formed from themselves.
Note: Only the most stable form of an element has a zero enthalpy of formation.
Ozone (O₃) is a less stable allotropic form of oxygen. Its enthalpy of formation is 142.7 kJ/mol.
Diamond is a less stable allotrope of carbon. Its enthalpy of formation is 1.9 kJ/mol.

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