Ionic Bonds in Reality
When two elements form an ionic compound, is an electron really lost by one atom and transferred to the other one? In order to answer this question, consider the data on the ionic solid LiF. The average radius of the neutral Li atom is about 2.52Å. If this Li atom reacts with an atom of F to form LiF, what is the average distance between the Li nucleus and the electron it has "lost" to the fluorine atom? The answer is 1.56Å; the electron is now closer to the lithium nucleus than it was in neutral lithium (Figure 1).
So the answer to the above question is both yes and no: yes, the electron that was now in the 2s orbital of Li is now within the grasp of a fluorine 2p orbital, but no, the electron is now even closer to the Li nucleus than before, so it is not truly "lost." The one thing that is inarguably true about LiF is that there are more electrons closer to positive nuclei than there are in the separated Li and F atoms. But this is just the rule we stated at the beginning of this unit: chemical bonds form when electrons can be simultaneously near two or more nuclei.
It is obvious that the electron-pair bond is responsible for this situation; this provides stability of the covalent bond. What is not so obvious—until you look at the numbers such as are quoted for LiF above—is that the “ionic” bond results in the same condition; even in the most highly ionic compounds, both electrons are close to both nuclei, and the resulting mutual attractions bind the nuclei together. This being the case, is there really any fundamental difference between the ionic and covalent bond?
The answer, according to modern chemical thinking, is probably no. In fact, there is some question as to whether it is realistic to consider that these solids consist of “ions” in the usual sense. The emerging view of ionic bonding is one in which the electron orbitals of adjacent atom pairs are simply skewed so as to place more electron density around the “negative” element than around the “positive” one. One can think of the magnitude of this skewing as the percent ionic character of a bond. To determine the percent ionic character, one must look at the electronegativities of the atoms involved and determine how effective the electron sharing is between the species.
This being said, the ionic model of bonding is useful for many purposes, and there is nothing wrong with using the term “ionic bond” to describe the interactions between the atoms in the very small class of “ionic solids” such as LiF and NaCl.
A bond angle is the angle formed between three atoms across at least two bonds. The more covalent in nature the bond, the more likely that the atoms will situate themselves along the predetermined vectors given by the orbitals that are involved in bonding. The more ionic character there is to a bond, the more likely that non-directional electrostatic interactions are holding the atoms together. This means atoms will sit in positions that minimize the amount of space they take up.