The standard enthalpy of formation of calcium chloride is -795kJmol-1, the equation for the reaction is as follows:Ca(s) + Cl2(g) → CaCl2(s)The definition states that the -795kJ is the enthalpy for 1 mole of calcium chloride. The definition of the enthalpy of atomisation is for the formation of 1 mole of calcium atoms and 1 mole of chlorine atoms but we need to form 2Cl(g) so the enthalpy change for the atomisation will be 2x +121 kJ mol-1 for chlorine. The definition of the enthalpy of atomisation is for the formation of 1 mole of calcium atoms so we can use the standard enthalpy of atomisation which is +178 kJmol-1 as it is. The next step is to ionise the calcium. The equation for the first ionisation energy is:Ca(g) → Ca+(g) + e-The first ionisation energy for calcium is +590 kJ per mole. The calcium ion has a +2 charge so a second ionisation energy (+1145 kJ per mole) is needed.Ca+(g) → Ca2+(g) + e-Mass and charge are conserved for each step. We now have 2x the electron affinity for chlorine.2Cl(g) + 2e- → 2Cl-(g)The cycle is completed with the Lattice (formation) enthalpyCa2+(g) + 2Cl-(g) → CaCl2 (s)