To break an HCl molecule into a hydrogen atom and a chlorine atom, we need to overcome the bond energy between the hydrogen and chlorine atoms. The bond energy of HCl is 431.7 kJ/mol. This means that it takes 431.7 kilojoules of energy to break one mole of HCl molecules into individual hydrogen and chlorine atoms.

Therefore, the minimum energy required to break an HCl molecule into a hydrogen atom is 431.7 kJ/mol.