2-18. The energy released when sodium and chlorine combine to form NaCl is 4.2 eV . (a) What is the increase in mass (in unified mass units) when a molecule of NaCl is dissociated into an atom of Na and an atom of Cl ? (b) What percentage of error is made in neglecting this mass difference? (The mass of Na is about 23 u and that of Cl is about 35.5 u .)