The standard enthalpy of formation (heat of formation) of MgO is -601.7 kJ/mol.

This means that when one mole of MgO is formed from its elements in their standard states (solid magnesium and gaseous oxygen), 601.7 kJ of heat is released. This reaction is exothermic because the enthalpy of the products (MgO) is lower than the enthalpy of the reactants (Mg and O2).