The ionic bond
The bond valence model is derived from the ionic model by examining the Coulomb field (Madelung field). Since ions have spherical symmetry, they can be approximated by point charges placed at their centre. Neighbouring ions are linked by electrostatic flux (lines of electric field). A bond is defined as existing between two ions that are linked by electrostatic flux, and the coordination number of an ion is the number of bonds so formed. Representing the bonds as links between ions (nodes) creates a bond network, and since bonds in the ionic model behave like electric capacitors, the electrostatic flux (bond valence) can be calculated using the Kirchhoff equations, providing that the bond capacitances are known. In many equilibrium structures all the bond capacitors are found to be equal and cancel out of the Kirchhoff equations. Methods for solving the Kirchhoff equations appear in Appendix 3.
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