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Ionic bonds are formed when two ions are held together by electrostatic attraction. Shown on the left above is an ionic bond between lithium and fluorine in Li-F. These atoms are plotted with electrostatic potential surfaces. That is fancy language for saying that the overall charge is plotted in color. The plot on the left shows the surface as solid, while the plot on the right is the same except the surface is transparent so you can see the nuclei inside. The red color indicates the negative charge of the fluoride anion, the blue charge indicates the positive charge of the lithium cation. The atoms are held together because opposite charges attract each other. This is an ionic bond. Contrast this to the situation with the fuorine molecule (F-F) shown on the right in which both atoms have the same charge, so there is only green color, no red or blue. The type of bond found in the fluorine molecule is called a covalent bond and comes about because the atoms share electron density in order to each obtain a noble gas configuration.
The figure above uses three types of electron density models to compare the bonding and polarity of simple hydrides: LiH, H2, and HF. The mesh surfaces identify points where the electron density is relatively low (0.002 a.u.). These points more or less define the "edge" of the electron "cloud" in each molecule. Notice how the size of the electron cloud near H shrinks as its bonding partner changes from Li -> H -> F. Recalling the analysis of Li and Li+ given in Figure 1, you can conclude that the amount of electron density belonging to H is greatest in LiH and least in HF. In other words, Li and F do not share bonding electrons equally with H (equal sharing must occur in H2). Li donates electron density to H, but F "steals" electron density from H.
Chemists describe the ability of an atom to "steal" bonding electrons from its partner as its electronegativity. These figures demonstrate that electronegativity increases in the order: Li -- H -- F.
The colored maps show how each molecule's electrostatic potential varies on the 0.002 isodensity surfaces. The variation in potential is shown by color - RED (lowest) -> ORANGE -> YELLOW -> GREEN -> BLUE (highest) - and indicates whether a particular region is electron-rich (RED) or electron-poor (BLUE). LiH and HF are polar molecules in that the two ends of these molecules are electron-rich and electron-poor respectively. The change in potential around H is also consistent with the previous analysis based on surface size; H is electron-rich in LiH (RED), neutral in H2 (GREEN), and electron-poor in HF (BLUE).
Finally, the solid surfaces (inside the mesh surfaces) identify points where the electron density is relatively high (0.08 a.u.). Atoms that share electrons (covalently bonded) build up electron density in the region between the two nuclei. The models of H2 and HF show that these molecules contain covalent bonds - the electron density between the nuclei is 0.08 a.u. or greater. The model of LiH, on the other hand, suggests that this bond is largely ionic - although there are regions of high electron density around each nucleus, electron density is less than 0.08 between the two nuclei.