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Polarization effects Ions in crystal lattices of purely ionic compounds are spherical, but, if the positive ion is small and/or highly charged, it will distort the electron cloud of the negative ion. 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 only important when positive ions with charges of 3+ (e.g., Al3+) are involved (e.g., pure AlCl3 is a covalent molecule). However, 2+ ions (Be2+) or even 1+ (Li+) show some polarizing power because their sizes are so small (e.g., LiI is ionic but has some covalent character). The Polarizing Power depends on the ratio of charge and size of the ion, often called the charge density. Ionic Structure Ionic compounds in the solid state form a continuous ionic lattice structure in an ionic crystal. When all the ions are approximately the same size, they can form a structure that is face-centered cubic, but, when the ions are different sizes, the structure is often body-centered cubic. In ionic lattices the coordination number refers to the number of ions that each is connected to. Ionic versus covalent bonds In an ionic bond, the atoms are bound by attraction of opposite ions, whereas, in a covalent bond, atoms are bound by sharing electrons. In covalent bonding, the molecular geometry around each atom is determined by VSEPR rules, whereas, in ionic materials, the geometry follows maximum packing rules. Thus, a compound can be classified as ionic or covalent based on the geometry of the atoms. See also | ||||||||||
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