This equation can be used to calculate bond enthalpies between two different elements (X and Y) using pauling electronegativities and the bond enthalpies between two atoms of those elements (X2 and Y2). Pauling electronegativities help calculate the square root of the ionicity of the X-Y bond which is equal to delta D. When experimental data of the bond enthalpies of for example D(X2)=I2 and D(Y2)=Br2 are provided there is then only one unknown the bond enthalpy of (X-Y) or (I-Br). This equation can be arranged to calculate any one of these variables as long as all the rest are known.
This equation is the Borne-Lande equation and is used to calculate lattice enthalpy from; NA-Avagadro's constant(6.022x1023mol-1),the Madelung constant which comes from the equation below (see equation 3), e2 which the charge of a proton(1.602x10-19C)2, Z+ is equal to the charge of the cations in the lattice, Z-is equal to the charge of the anion in the lattice, epsilon 0 is equal to the permittivity of a vacuum(8.854x10-12m-3Kg-1s4A2),r0is equal to the ionic radius which must be converted to metres.
This is an equation to explain how the madelung constant is calculated. The madelung constant is calculated from the attractive and repulsive forces within the cations and anions within an ionic lattice. The bottom half of every fraction is the square root of how many layers it is from considering the first layer of ions.
This is an equation to calculate the ionicity of a the bond X-Y. As one of the atoms in the bond will be more elctronegative than the other so it is more electron withdrawing. The ionicity must be considered when calculating the strength of a bond (see equation one)