A Born-Haber cycle is a type of Hess's Law energy cycle. Born-Haber cycles allow the enthalpy changes which cannot be measured directly to be calculated from other enthalpy changes. Elements in their standard state start the cycle on the datum line. The next step is to add the enthalpy of formation. Arrows going downwards signify exothermic reactions. Arrows going upwards signify endothermic reactions. Each of the elements are then atomised in turn. This can be done in any order. Electrons are removed from the metal to make it into a positive gaseous ion. Electrons are added to the non-metal to make it a negative gaseous ion. The ions bond to form an ionic lattice.Lattice enthalpy is always exothermic, so the arrow always points downwards.