The 1st1^{\text{st}}1stionization enthalpy for Mg is +737 kJ/mol+737\ \text{kJ/mol}+737 kJ/mol. The most probable estimated value of the 2nd2^{\text{nd}}2nd ionization enthalpy of Mg is ____.
−906 kJ/mol-906\ \text{kJ/mol}−906 kJ/mol
−856 kJ/mol-856\ \text{kJ/mol}−856 kJ/mol
+1450 kJ/mol+1450\ \text{kJ/mol}+1450 kJ/mol
+590 kJ/mol+590\ \text{kJ/mol}+590 kJ/mol