0.9 Energy and polarity of covalent chemical bonds  (Page 2/8)

Once we have an understanding of what sharing electrons means, we will also ask whether the atoms really do “share” these electrons fully. We already know that the energies of electrons in different atoms are quite different. We have observed and analyzed the ionization energies of electrons in atoms. These results showed that the electrons in some atoms such as fluorine, for example, are much more strongly attracted to their nuclei than in atoms such as carbon, for example. Knowing this, we might readily ask whether the electrons shared by a C atom bonded to an F atom are actually shared equally. If they are not equally shared, does it matter? Does this affect the physical or chemical properties of the molecules?

As always, we will examine experimental observations to help us understand the answers to these questions.

Foundation

In this Concept Development Study, we will assume that we already know the basic rules of the Lewis model of chemical bonding. Chemical bonds between atoms consist of one, two, or three pairs of shared electrons, respectively resulting in single, double, or triple bonds. Atoms in groups IV, V, VI, and VII, most importantly including C, N, O, and F, share electrons in pairs such that, in stable molecules, these atoms typically have eight electrons in their valence shells. An H atom will share one pair of electrons with other types of atoms to form stable molecules.

We will need Coulomb’s Law to understand how the bonding electrons interact with the nuclei of the atoms that share them. We have already used Coulomb’s Law to understand the energies of electrons in atoms and how these vary from one type of atom to another. These same lines of reasoning will be useful for electrons in molecules.

Though we will not need all of the postulates and conclusions of Quantum Mechanics, there are a few that we will call on to answer the questions posed above. This makes sense, since any discussion of where electrons might be found or how they might move certainly requires us to take a quantum mechanical view of electrons. Most notably, we’ll need to recall that the motion of electron is described by an “orbital” which provides the probability for where the electron might be. In addition, we’ll recall that an electron can exist in two different “spin states,” and that two electrons’ motions may be described by the same orbital only if they have different spin states.

Observation 1: the simplest chemical bond, h ₂ ⁺

Let’s start with the easiest molecule we can imagine. We need at least two atoms of course, so that means we need two nuclei. The smallest nucleus is just a single proton, in other words, a hydrogen ion, H ⁺ . We could try to build a molecule formed from just two of these, but this seems unlikely. The two positively charged nuclei would just repel each other, so there can’t be such a molecule. We need at least one electron to provide some attraction. Is one enough? The best way to answer that question is to ask whether the combination of two H ⁺ nuclei plus one electron is a stable molecule, H ₂ ⁺ .