<P> Ionic compounds in the solid state form lattice structures . The two principal factors in determining the form of the lattice are the relative charges of the ions and their relative sizes . Some structures are adopted by a number of compounds; for example, the structure of the rock salt sodium chloride is also adopted by many alkali halides, and binary oxides such as MgO . Pauling's rules provide guidelines for predicting and rationalizing the crystal structures of ionic crystals </P> <P> For a solid crystalline ionic compound the enthalpy change in forming the solid from gaseous ions is termed the lattice energy . The experimental value for the lattice energy can be determined using the Born - Haber cycle . It can also be calculated (predicted) using the Born - Landé equation as the sum of the electrostatic potential energy, calculated by summing interactions between cations and anions, and a short - range repulsive potential energy term . The electrostatic potential can be expressed in terms of the inter-ionic separation and a constant (Madelung constant) that takes account of the geometry of the crystal . The further away from the nucleus the weaker the shield . The Born - Landé equation gives a reasonable fit to the lattice energy of, e.g., sodium chloride, where the calculated (predicted) value is − 756 kJ / mol, which compares to − 787 kJ / mol using the Born - Haber cycle . </P> <P> Ions in crystal lattices of purely ionic compounds are spherical; however, if the positive ion is small and / or highly charged, it will distort the electron cloud of the negative ion, an effect summarised in Fajans' rules . This polarization of the negative ion leads to a build - up of extra charge density between the two nuclei, i.e., to partial covalency . Larger negative ions are more easily polarized, but the effect is usually important only when positive ions with charges of 3 + (e.g., Al) are involved . However, 2 + ions (Be) or even 1 + (Li) show some polarizing power because their sizes are so small (e.g., LiI is ionic but has some covalent bonding present). Note that this is not the ionic polarization effect that refers to displacement of ions in the lattice due to the application of an electric field . </P> <P> In ionic bonding, the atoms are bound by attraction of oppositely charged ions, whereas, in covalent bonding, atoms are bound by sharing electrons to attain stable electron configurations . In covalent bonding, the molecular geometry around each atom is determined by valence shell electron pair repulsion VSEPR rules, whereas, in ionic materials, the geometry follows maximum packing rules . One could say that covalent bonding is more directional in the sense that the energy penalty for not adhering to the optimum bond angles is large, whereas ionic bonding has no such penalty . There are no shared electron pairs to repel each other, the ions should simply be packed as efficiently as possible . This often leads to much higher coordination numbers . In NaCl, each ion has 6 bonds and all bond angles are 90 degrees . In CsCl the coordination number is 8 . By comparison carbon typically has a maximum of four bonds . </P>

An ionic compound is held together by ionic bonds